1031-20/10

017. Letter Combinations of a Phone Number.py

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+# Given a digit string, return all possible letter combinations that the number could represent.
+
+# A mapping of digit to letters (just like on the telephone buttons) is given below.
+
+
+
+# Input:Digit string "23"
+# Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
+
+# DFS backtracking
+class Solution(object):
+    def letterCombinations(self, digits):
+        """
+        O(2^n)
+        :type digits: str
+        :rtype: List[str]
+        """
+        if not digits:
+            return []
+        res = []
+        dic = {
+            '2': 'abc',
+            '3': 'def',
+            '4': 'ghi',
+            '5': 'jkl',
+            '6': 'mno',
+            '7': 'pqrs',
+            '8': 'tuv',
+            '9': 'wxyz'
+        }
+        self.dfs(res, '',dic,digits,0)
+        return res
+
+    def dfs(self, res, temp, dic,digits,num):
+        if num == len(digits):
+            res.append(temp)
+            return
+
+        # focus on !
+        for letter in dic[digits[num]]:
+            self.dfs(res, temp+letter, dic, digits, num+1)
+
+
+

173. Binary Search Tree Iterator.py

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+# Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.
+
+# Calling next() will return the next smallest number in the BST.
+
+# Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.
+
+# Credits:
+# Special thanks to @ts for adding this problem and creating all test cases.
+
+# Definition for a  binary tree node
+# class TreeNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class BSTIterator(object):
+    def __init__(self, root):
+        """
+        :type root: TreeNode
+        """
+        self.stack = []
+        self.push(root)        
+
+
+    def hasNext(self):
+        """
+        :rtype: bool
+        """
+        return True if self.stack else False
+
+
+    def next(self):
+        """
+        :rtype: int
+        """
+        node = self.stack.pop()
+        self.push(node.right)
+        return node.val
+
+    def push(self,node):
+        while node:
+        	# imp
+            self.stack.append(node)
+            node = node.left
+
+
+# Your BSTIterator will be called like this:
+# i, v = BSTIterator(root), []
+# while i.hasNext(): v.append(i.next())