diff --git "a/\351\231\210\344\271\220\345\244\251/README.md" "b/\351\231\210\344\271\220\345\244\251/README.md"
index a128615..af308b8 100644
--- "a/\351\231\210\344\271\220\345\244\251/README.md"
+++ "b/\351\231\210\344\271\220\345\244\251/README.md"
@@ -12,4 +12,8 @@
 - 八皇后那题做得好丑。。。完全就是用递归替代循环写了一遍八皇后的dfs，写到第六题的时候突然发现八皇后那题应该用第六题那种方法写，但是好像想想似乎构造出来长度为8的序列之后好像还是得用类似于两重循环的递归去判断是否符合呀（且不说长度为8的序列写出来我已经画美不看了），求指教，如何用优雅的方式写八皇后那题
 - 感觉这周终于有点函数式的感觉了😂再接再厉！
 
+## Week4
+- 第四周作业题已全部AC
+- 画小人那个确实蛮有意思，绘图质量这么高我竟然有点在考虑用racket做生产力工具了（卧槽这东西写出来是不是永远别想维护了），要是有个好的IDE真的可能会选择racket
+
 
diff --git "a/\351\231\210\344\271\220\345\244\251/Week4/graphic.rkt" "b/\351\231\210\344\271\220\345\244\251/Week4/graphic.rkt"
new file mode 100644
index 0000000..1ef32e2
--- /dev/null
+++ "b/\351\231\210\344\271\220\345\244\251/Week4/graphic.rkt"
@@ -0,0 +1,215 @@
+#lang racket/gui
+(require racket/gui/base)
+
+;定义向量
+(define (make-vect x y) (cons x y))
+(define (xcor-vect v)(car v))
+(define (ycor-vect v)(cdr v))
+(define (add-vect v1 v2)
+  (make-vect (+ (xcor-vect v1)
+                (xcor-vect v2))
+             (+ (ycor-vect v1) (ycor-vect v2))))
+(define (sub-vect v1 v2)
+  (make-vect (- (xcor-vect v1)
+                (xcor-vect v2)) (- (ycor-vect v1)
+                                   (ycor-vect v2))))
+(define (scale-vect s v)
+  (make-vect (* s (xcor-vect v))
+             (* s (ycor-vect v))))
+;定义线段
+(define (make-segment start end) (cons start end))
+(define (start-segment seg) (car seg))
+(define (end-segment seg) (cdr seg))
+(define (scale-segments seg-list scale)
+  (map
+   (lambda (segment)
+     (cons (scale-vect scale (start-segment segment)) (scale-vect scale (end-segment segment))))
+   seg-list))
+;start, end都是线段端点(向量),坐标相对于绝对原点
+
+;定义frame
+(define (make-frame origin edge1 edge2)
+  (list origin edge1 edge2)) ;edge1和edge2也可以看做点,其坐标是相对于frame原点的
+(define (origin-frame f)  (car f))
+(define (edge1-frame f)  (cadr f))
+(define (edge2-frame f)  (caddr f))
+;向量转换器
+(define (frame-coord-map frame)
+  (lambda (v)
+    (add-vect
+     (origin-frame frame)
+     (add-vect (scale-vect (xcor-vect v)
+                           (edge1-frame frame))
+               (scale-vect (ycor-vect v)
+                           (edge2-frame frame))))))
+;定义painter
+(define (segments->painter segment-list) ;segment-list是线段列表
+  (lambda (frame)
+    (for-each
+     (lambda (segment)
+       (draw-line ;假定draw-line可以画线(以绝对原点作为原点)
+        ((frame-coord-map frame) (start-segment segment))
+        ((frame-coord-map frame) (end-segment segment))))
+     segment-list))) ;本过程生成一个painter,其原图形是一系列线段
+;accumulate
+(define (accumulate op initial sequence)
+  (if (null? sequence)
+      initial
+      (op (car sequence)
+          (accumulate op initial (cdr sequence)))))
+;flatmap
+(define (flatmap proc seq)
+  (accumulate append '() (map proc seq)))
+;将点组成的集合的集合转化为线段
+(define (point->segments lst)
+  (flatmap
+   (lambda (point-list)
+     (define (getSegment points)
+       (if (null? (cdr points))
+           '()
+           (cons (make-segment (car points) (cadr points)) (getSegment (cdr points)))))
+     (getSegment point-list))
+   lst))
+;转换painter
+(define (transform-painter painter origin corner1 corner2)
+   (lambda (frame)
+     (let ((m (frame-coord-map frame)))
+       (let ((new-origin (m origin)))
+         (painter
+          (make-frame new-origin
+                      (sub-vect (m corner1) new-origin);frame的两条边是相对于frame原点的
+                      (sub-vect (m corner2) new-origin)))))))
+;一系列转换painter的方法
+;恒等
+(define (identity painter) painter)
+;水平翻转
+(define (flip-horz painter)
+  (transform-painter painter
+                     (make-vect 1 0)
+                     (make-vect 0 0)
+                     (make-vect 1 1)))
+;竖直翻转
+(define (flip-vert painter)
+  (transform-painter painter
+                     (make-vect 0 1)
+                     (make-vect 1 1)
+                     (make-vect 0 0)))
+;旋转180度
+(define (rotate180 painter)
+  (transform-painter painter
+                     (make-vect 1 1)
+                     (make-vect 0 1)
+                     (make-vect 1 0)))
+;上下排列
+(define (below painter1 painter2)
+  (let ((up (transform-painter painter1
+                               (make-vect 0 0)
+                               (make-vect 1 0)
+                               (make-vect 0 0.5)))
+        (down (transform-painter painter2
+                                 (make-vect 0 0.5)
+                                 (make-vect 1 0.5)
+                                 (make-vect 0 1))))
+    (lambda (frame)
+      (up frame)
+      (down frame))))
+;左右排列
+(define (beside painter1 painter2)
+  (let ((left (transform-painter painter1
+                               (make-vect 0 0)
+                               (make-vect 0.5 0)
+                               (make-vect 0 1)))
+        (right (transform-painter painter2
+                                 (make-vect 0.5 0)
+                                 (make-vect 1 0)
+                                 (make-vect 0.5 1))))
+    (lambda (frame)
+      (left frame)
+      (right frame))))
+;在右侧递归排列
+(define (right-split painter n) ;生成新painter,右分n次
+  (if (= n 0)
+      painter
+      (let ((smaller (right-split painter (- n 1))))
+        (beside painter (below smaller smaller)))))
+;在上侧递归排列
+(define (up-split painter n) ;生成新painter,右分n次
+  (if (= n 0)
+      painter
+      (let ((smaller (up-split painter (- n 1))))
+        (below (beside smaller smaller) painter))))
+;在右上递归排列
+(define (corner-split painter n)
+  (if (= n 0)
+      painter
+      (let ((up (up-split painter (- n 1)))
+            (right (right-split painter (- n 1))))
+        (let ((top-left (beside up up))
+             (bottom-right (below right right))
+             (top-right (corner-split painter (- n 1))))
+          (beside (below top-left painter) (below top-right bottom-right))))))
+;将四个方块结合
+(define (square-of-four tl tr bl br)
+  (lambda (painter)
+    (let ((top (beside (tl painter) (tr painter))) (bottom (beside (bl painter) (br painter))))
+      (below top bottom))))
+;square-limit
+(define (square-limit painter n)
+  (let ((combine4 (square-of-four flip-horz identity
+                                  rotate180 flip-vert)))
+    (combine4 (corner-split painter n))))
+
+;drawLine
+(define linesToDraw '())
+(define (draw-line a b)
+  (set! linesToDraw (cons (make-segment a b) linesToDraw)))
+
+;处理小人的数据
+(define figure
+  (list
+   (list (make-vect 0 26)
+         (make-vect 6 17)
+         (make-vect 12 25)
+         (make-vect 14 21)
+         (make-vect 10 0))
+   (list (make-vect 16 0)
+         (make-vect 21 13)
+         (make-vect 25 0))
+   (list (make-vect 31 0)
+         (make-vect 25 19)
+         (make-vect 41 6))
+   (list (make-vect 41 15)
+         (make-vect 31 27)
+         (make-vect 25 27)
+         (make-vect 27 35)
+         (make-vect 25 41))
+   (list (make-vect 16 41)
+         (make-vect 14 35)
+         (make-vect 16 27)
+         (make-vect 12 27)
+         (make-vect 6 25)
+         (make-vect 0 35))))
+(define figure-segments (scale-segments (point->segments figure) 0.024))
+
+;开始画图
+(define wave (flip-vert (segments->painter figure-segments)))
+(define frame1 (make-frame (make-vect 0 0) (make-vect 400 0) (make-vect 0 400)))
+((square-limit wave 4) frame1)
+(define frame2 (make-frame (make-vect 500 0) (make-vect 300 50) (make-vect 150 300)))
+((beside wave (flip-horz (rotate180 wave))) frame2)
+
+;创建窗体及画布
+(define frame (new frame% [label "萌萌的小人"] [width 1000] [height 500]))
+(new canvas% [parent frame]
+             [paint-callback
+              (lambda (canvas dc)
+                (send dc set-pen "red" 1 'solid)
+                (for-each
+                 (lambda (line)
+                   (let ((p1 (start-segment line))
+                         (p2 (end-segment line)))
+                     (send dc draw-line
+                           (xcor-vect p1) (ycor-vect p1)
+                           (xcor-vect p2) (ycor-vect p2))))
+                 linesToDraw))])
+(send frame show #t)
\ No newline at end of file
diff --git "a/\351\231\210\344\271\220\345\244\251/Week4/homework4-1.rkt" "b/\351\231\210\344\271\220\345\244\251/Week4/homework4-1.rkt"
new file mode 100644
index 0000000..885bb49
--- /dev/null
+++ "b/\351\231\210\344\271\220\345\244\251/Week4/homework4-1.rkt"
@@ -0,0 +1,34 @@
+#lang racket
+
+(define (min a b)
+  (if (< a b)
+      a
+      b))
+(define (doWork a b result)
+  (define (find-min x nowmin now)
+    (if (= now (- (length x) 1))
+        (min nowmin (list-ref x now))
+        (if (< (list-ref x now) nowmin)
+            (find-min x (list-ref x now) (+ now 1))
+            (find-min x nowmin (+ now 1)))))
+  (cond ((and (empty? a) (empty? b)) (displayln result))
+        ((empty? a)
+         (define minb (find-min b (car b) 0))
+         (define minNumber minb)
+         (doWork (filter (lambda (num) (not (= num minNumber))) a) (filter (lambda (num) (not (= num minNumber))) b) (append result (list minNumber))))
+        ((empty? b)
+         (define mina (find-min a (car a) 0))
+         (define minNumber mina)
+         (doWork (filter (lambda (num) (not (= num minNumber))) a) (filter (lambda (num) (not (= num minNumber))) b) (append result (list minNumber))))
+        (else
+         (define mina (find-min a (car a) 0))
+         (define minb (find-min b (car b) 0))
+         (define minNumber (min mina minb))
+         (doWork (filter (lambda (num) (not (= num minNumber))) a) (filter (lambda (num) (not (= num minNumber))) b) (append result (list minNumber))))))
+(define (myloop)
+  (let ((a (read))
+        (b (read)))
+    (if (eq? a eof)
+        (void)
+        (begin (doWork a b (list)) (myloop)))))
+(myloop)
diff --git "a/\351\231\210\344\271\220\345\244\251/Week4/homework4-2.rkt" "b/\351\231\210\344\271\220\345\244\251/Week4/homework4-2.rkt"
new file mode 100644
index 0000000..0309f06
--- /dev/null
+++ "b/\351\231\210\344\271\220\345\244\251/Week4/homework4-2.rkt"
@@ -0,0 +1,128 @@
+#lang racket
+
+(define (generate-huffman-tree lst)
+  (define leaf-set (make-leaf-set lst))
+  (define (generater node)
+    (if (null? (cdr node))
+        (car node)
+        (begin
+          (let ((one (car node))
+                (two (cadr node)))
+            (define newNode (make-code-tree one two))
+            (generater (adjoin-set newNode (cddr node)))))))
+  (generater leaf-set))
+
+(define (encode str tree)
+  (define (isin? x tree)
+    (define sym (symbols tree))
+    (define (find x syms)
+      (if (null? syms)
+          #f
+          (if (equal? (symbol->string (car syms)) (string x))
+              #t
+              (find x (cdr syms)))))
+    (find x sym))
+  (define (encode-1 codes sub-tree)
+    (if (null? codes)
+        '()
+        (if (leaf? sub-tree)
+        (encode-1 (cdr codes) tree)
+        (if (isin? (car codes) (left-branch sub-tree))
+            (cons 0 (encode-1 codes (left-branch sub-tree)))
+            (cons 1 (encode-1 codes (right-branch sub-tree)))))))
+  (define charlist (string->list (symbol->string str)))
+  (encode-1 charlist tree))
+
+
+(define (make-leaf symbol weight)
+  (list 'leaf symbol weight))
+
+(define (leaf? object)
+  (eq? (car object) 'leaf))
+
+(define (symbol-leaf x) (cadr x))
+(define (weight-leaf x) (caddr x))
+
+(define (make-code-tree left right)
+  (list left 
+        right
+        (append (symbols left ) (symbols right))
+        (+ (weight left) (weight right))))
+
+(define (left-branch tree) (car tree))
+(define (right-branch tree) (cadr tree))
+(define (symbols  tree)
+  (if (leaf? tree) 
+      (list (symbol-leaf tree))
+      (caddr tree)))
+
+(define (weight tree)
+  (if (leaf? tree) (weight-leaf tree)
+      (cadddr tree)))
+
+  
+  
+(define (decode bits tree)
+  (define (decode-1 bits current-branch)
+    (if (null? bits)
+        '()
+        (let ((next-branch
+               (choose-branch (car bits) current-branch)))
+          (if (leaf? next-branch)
+              (cons (symbol-leaf next-branch)
+                    (decode-1 (cdr bits) tree))
+              (decode-1 (cdr bits) next-branch)))))
+  (decode-1 bits tree))
+
+(define (choose-branch bit branch)
+  (cond ((= bit 0) (left-branch branch))
+        ((= bit 1) (right-branch branch))
+        (else (error "bad bit " bit))))
+
+
+(define (adjoin-set x set)
+;  (display "in adjoin-set:" ) (display "x=") (display x) (display "  set=" ) (display set) (newline);addfor debug
+  (cond ((null? set) (list x))
+        ((< (weight x) (weight (car set))) (cons x set))
+        (else (cons (car set)
+                    (adjoin-set x (cdr set))))))
+
+
+(define (make-leaf-set pairs)
+;  (display "in make-leaf-set:" ) (display pairs) (newline) ;addfor debug
+  (if (null? pairs)
+      '()
+      (let ((pair (car pairs)))
+        (adjoin-set (make-leaf (car pair)
+                               (cadr pair))
+                    (make-leaf-set (cdr pairs))))))
+
+
+;(define (my-number->list num)
+;  (if (< num 10)
+;      (cons num '())
+;      (append (my-number->list (floor (/ num 10))) (list (remainder num 10)))))
+;(define tmp '((A 10000000) (B 1000000) (C 100000) (D 10000) (E 1000) (F 100) (G 10) (H 1)))
+;(define tmptmp (make-leaf-set tmp))
+;(define mytree (generate-huffman-tree tmp))
+;(encode 'ABEFG mytree)
+
+
+(define huffman-tree '())
+(define (myloop) 
+  (define (display-list lst)
+    (if (null? lst)
+        (void)
+        (begin (display (car lst)) (display-list (cdr lst)))))
+
+  (let ((a (read)))
+     (if (eq? a eof)
+         (void)
+         (cond ((eq? a 'B) 
+                (set! huffman-tree (generate-huffman-tree (read))) (myloop))
+               ((eq? a 'E) 
+                (display-list (decode (encode (read) huffman-tree) huffman-tree))
+                (newline)
+                (myloop))))))
+
+(myloop)
\ No newline at end of file
diff --git "a/\351\231\210\344\271\220\345\244\251/Week4/homework4-3.rkt" "b/\351\231\210\344\271\220\345\244\251/Week4/homework4-3.rkt"
new file mode 100644
index 0000000..2f30978
--- /dev/null
+++ "b/\351\231\210\344\271\220\345\244\251/Week4/homework4-3.rkt"
@@ -0,0 +1,98 @@
+#lang racket
+
+
+(define (createList a)
+  (if (list? a)
+      a
+      (list a)))
+
+(define (variable? exp) (symbol? exp))
+
+(define (same-variable? v1 v2)
+  (and (variable? v1) (variable? v2) (eq? v1 v2)))
+
+(define (sum? exp)
+  (define (findPlus now)
+    (if (null? now)
+        #f
+        (if (eq? (car now) '+)
+            #t
+            (findPlus (cdr now)))))
+  (if (pair? exp)
+      (findPlus exp)
+      #f))
+
+(define (make-sum a1 a2)
+  (cond ((=number? a1 0) a2)
+        ((=number? a2 0) a1)
+        ((and (number? a1) (number? a2)) (+ a1 a2)) (else (append (createList a1) (list '+) (createList a2)))))
+
+(define (addend exp)
+  (define (plusBefore now)
+    (if (eq? (car now) '+)
+        '()
+        (cons (car now) (plusBefore (cdr now)))))
+  (define beforeExp (plusBefore exp))
+  (if (null? (cdr beforeExp))
+      (car beforeExp)
+      beforeExp))
+
+(define (augend exp)
+  (define (plusAfter now)
+    (define (plusAfterReally now)
+      (if (null? now)
+          '()
+          (cons (car now) (plusAfterReally (cdr now)))))
+    (if (eq? (car now) '+)
+        (plusAfterReally (cdr now))
+        (plusAfter (cdr now))))
+  (define afterExp (plusAfter exp))
+  (if (null? (cdr afterExp))
+      (car afterExp)
+      afterExp))
+
+(define (product? exp) (and (pair? exp) (eq? (cadr exp) '*)))
+
+(define (make-product a1 a2)
+  (cond ((or (=number? a1 0) (=number? a2 0)) 0)
+        ((=number? a1 1) a2)
+        ((=number? a2 1) a1)
+        ((and (number? a1) (number? a2)) (* a1 a2)) (else (list a1 '* a2))))
+
+(define (multiplicand exp)
+  (if (null? (cdddr exp))
+      (caddr exp)
+      (cddr exp)))
+
+(define (multiplier exp) (car exp))
+
+(define (=number? a b)
+  (if (number? a)
+      (if (= a b)
+          #t
+          #f)
+      #f))
+
+(define (deriv exp var)
+  (cond ((number? exp ) 0)
+        ((variable? exp)
+         (if (same-variable? exp var) 1 0))
+        ((sum? exp)
+         (make-sum (deriv (addend exp) var)
+                   (deriv (augend exp) var)))
+        ((product? exp)
+         (make-sum 
+          (make-product (multiplier exp)
+                        (deriv (multiplicand exp) var))
+          (make-product (deriv (multiplier exp) var)
+                        (multiplicand exp))))
+        (else 
+         (error "wrong format"))))
+
+(define (myloop)
+  (let ((a (read)))
+    (if (eq? a eof)
+        (void)
+        (begin (display (deriv a 'x)) (newline) (myloop)))))
+
+(myloop)
\ No newline at end of file