feat: 230. Kth Smallest Element in a BST

Author: gwbaik9717

kth-smallest-element-in-a-bst/gwbaik9717.js

@@ -0,0 +1,128 @@
+// Time complexity: O(nlogn)
+// Space complexity: O(n)
+
+class _Queue {
+  constructor() {
+    this.q = [];
+    this.front = 0;
+    this.rear = 0;
+  }
+
+  push(value) {
+    this.q.push(value);
+    this.rear++;
+  }
+
+  shift() {
+    const rv = this.q[this.front];
+    delete this.q[this.front++];
+    return rv;
+  }
+
+  isEmpty() {
+    return this.front === this.rear;
+  }
+}
+
+class MinHeap {
+  constructor() {
+    this.heap = [null];
+  }
+
+  push(value) {
+    this.heap.push(value);
+
+    let current = this.heap.length - 1;
+    let parent = Math.floor(current / 2);
+
+    while (parent && this.heap[current] < this.heap[parent]) {
+      [this.heap[current], this.heap[parent]] = [
+        this.heap[parent],
+        this.heap[current],
+      ];
+      current = parent;
+      parent = Math.floor(current / 2);
+    }
+  }
+
+  pop() {
+    if (this.heap.length === 1) {
+      return null;
+    }
+
+    if (this.heap.length === 2) {
+      return this.heap.pop();
+    }
+
+    const rv = this.heap[1];
+    this.heap[1] = this.heap.pop();
+
+    let current = 1;
+    let left = current * 2;
+    let right = left + 1;
+
+    while (
+      (this.heap[left] && this.heap[current] > this.heap[left]) ||
+      (this.heap[right] && this.heap[current] > this.heap[right])
+    ) {
+      if (this.heap[right] && this.heap[right] < this.heap[left]) {
+        [this.heap[right], this.heap[current]] = [
+          this.heap[current],
+          this.heap[right],
+        ];
+        current = right;
+      } else {
+        [this.heap[left], this.heap[current]] = [
+          this.heap[current],
+          this.heap[left],
+        ];
+        current = left;
+      }
+
+      left = current * 2;
+      right = left + 1;
+    }
+
+    return rv;
+  }
+}
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @param {number} k
+ * @return {number}
+ */
+var kthSmallest = function (root, k) {
+  const minHeap = new MinHeap();
+
+  const q = new _Queue();
+  q.push(root);
+
+  while (!q.isEmpty()) {
+    const current = q.shift();
+
+    minHeap.push(current.val);
+
+    if (current.left) {
+      q.push(current.left);
+    }
+
+    if (current.right) {
+      q.push(current.right);
+    }
+  }
+
+  let answer = null;
+  for (let i = 0; i < k; i++) {
+    answer = minHeap.pop();
+  }
+
+  return answer;
+};