[eunhwa99] Week13 Solutions

counting-bits/eunhwa99.kt

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+class Solution {
+    /*
+    0 -> 0
+    1 -> 1
+    2 -> 1
+    3 -> 2
+    4 -> 1
+    5 -> 2
+    6 -> 2
+    7 -> 3
+    8 -> 1
+    9 -> 2
+    10 -> 2
+    11 -> 3
+    12 -> 2
+    2,4,8,16,.. -> 1
+    짝수: i/2의 1의 개수
+    홀수: i/2의 1의 개수 + 1
+     */
+    // TC: O(n)
+    // SC: O(n)
+    fun countBits(n: Int): IntArray {
+        val result = IntArray(n + 1)
+        for(i in 0..n) {
+            result[i] = result[i / 2] + (i % 2)
+        }
+        return result
+    }
+}

find-median-from-data-stream/eunhwa99.kt

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+import java.util.PriorityQueue
+class MedianFinder() {
+    // 1 2 3 4 5
+    // 중간값을 기준으로 
+    //   3
+    //  2 4
+    // 1   5
+    // 1 2 : maxHeap
+    // 4 5 : minHeap
+    private val maxHeap = PriorityQueue<Int> { a, b -> b - a }
+    private val minHeap = PriorityQueue<Int>()
+
+    // TC: O(log n)
+    // SC: O(n)
+    fun addNum(num: Int) {
+       if(maxHeap.isEmpty() || maxHeap.peek() >= num) {
+        maxHeap.add(num)
+       } else {
+        minHeap.add(num)
+       }
+
+       if(maxHeap.size > minHeap.size + 1) {
+        minHeap.add(maxHeap.poll())
+       } else if(minHeap.size > maxHeap.size) {
+        maxHeap.add(minHeap.poll())
+       }
+    }
+
+    fun findMedian(): Double {
+       if(maxHeap.size == minHeap.size) {
+        return (maxHeap.peek() + minHeap.peek()) / 2.0
+       } else {
+        return maxHeap.peek().toDouble()
+       }
+    }
+
+}

insert-interval/eunhwa99.kt

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+class Solution {
+    fun insert(intervals: Array<IntArray>, newInterval: IntArray): Array<IntArray> {
+        val result = mutableListOf<IntArray>()
+        var currentStart = newInterval[0]
+        var currentEnd = newInterval[1]
+        var isAdded = false
+
+        // TC: O(n)
+        // SC: O(n)
+        for(interval in intervals){
+            if(isAdded) {
+                result.add(interval)
+            } else {
+                if(interval[1] < currentStart){ // new interval is after the current interval
+                    result.add(interval) // just add the interval without any changes
+                }
+                else if(interval[0] > currentEnd){ // new interval is before the current interval
+                    result.add(intArrayOf(currentStart, currentEnd))
+                    result.add(interval)
+                    isAdded = true
+                }
+                else{ // intervals overlap, merge them
+                    currentStart = minOf(interval[0], currentStart)
+                    currentEnd = maxOf(interval[1], currentEnd)
+                }
+            }
+        }
+
+        // If new interval wasn't added yet, add it now
+        if(!isAdded) {
+            result.add(intArrayOf(currentStart, currentEnd))
+        }
+
+        return result.toTypedArray()
+    }
+}

kth-smallest-element-in-a-bst/eunhwa99.kt

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+class Solution {
+    // TC : O(H+K)
+    // SC: O(H)
+    private var K: Int = 0
+    fun kthSmallest(root: TreeNode?, k: Int): Int {
+        K = k
+        return traverse(root)!!.`val`
+    }
+
+    fun traverse(cur: TreeNode?): TreeNode?{
+        if(cur == null) return cur
+
+        val result = traverse(cur.left)
+        K--
+        if(K == 0) return cur
+        return result ?: traverse(cur.right)
+    }
+}

lowest-common-ancestor-of-a-binary-search-tree/eunhwa99.kt

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+class TreeNode(var `val`: Int = 0) {
+    var left: TreeNode? = null
+    var right: TreeNode? = null
+}
+
+
+class Solution {
+
+    // TC: O(H) - H: height of BST
+    // SC: O(1) - while loop: X stack
+    fun lowestCommonAncestor(root: TreeNode?, p: TreeNode?, q: TreeNode?): TreeNode? {
+
+        if (root == null || p == null || q == null) return null
+
+        // BST: left < parent < right
+        var current = root
+        while(current != null){ 
+            if(current.`val` < p.`val` && current.`val` < q.`val`){
+                current = current.right
+            }
+            else if(current.`val` > p.`val` && current.`val` > q.`val`){
+                current = current.left
+            }
+            else return current
+        }
+
+        return null
+    }
+
+}

meeting-rooms/eunhwa99.java

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+
+// TC: O(N)
+// SC: O(1)
+class Solution {
+    /**
+     * @param intervals: an array of meeting time intervals
+     * @return: if a person could attend all meetings
+     */
+    public boolean canAttendMeetings(List<Interval> intervals) {
+        // Edge case: empty list or single meeting
+        if (intervals == null || intervals.size() <= 1) {
+            return true;
+        }
+
+        intervals.sort(Comparator.comparing(v->v.start));
+
+        int prevEndTime = intervals.get(0).end;
+        for(int i = 1; i < intervals.size(); i++){  
+            Interval cur = intervals.get(i);
+            if(cur.start < prevEndTime) return false;
+            prevEndTime = cur.end;
+        }
+        return true;
+    }
+}
+