https://leetcode.cn/problems/construct-binary-tree-from-inorder-and-postorder-traversal/

Author: masx200

README.md

@@ -10,6 +10,8 @@ leetcode 测试
 
 ##### 包含的内容如下
 
+https://leetcode.cn/problems/construct-binary-tree-from-inorder-and-postorder-traversal/
+
 https://leetcode.cn/problems/number-of-arithmetic-triplets/
 
 https://leetcode.cn/problems/7p8L0Z/

construct-binary-tree-from-inorder-and-postorder-traversal/index.ts

@@ -0,0 +1,40 @@
+import { TreeNode } from "../binary-tree-inorder-traversal/TreeNode.ts";
+
+export default function buildTree(
+    inorder: number[],
+    postorder: number[],
+): TreeNode | null {
+    if (inorder.length === 0) return null;
+    let post_idx = postorder.length - 1;
+    const idx_map = new Map<number, number>();
+    function helper(in_left: number, in_right: number) {
+        // 如果这里没有节点构造二叉树了，就结束
+        if (in_left > in_right) {
+            return null;
+        }
+
+        // 选择 post_idx 位置的元素作为当前子树根节点
+        const root_val = postorder[post_idx];
+        const root = new TreeNode(root_val);
+
+        // 根据 root 所在位置分成左右两棵子树
+        const index = idx_map.get(root_val) ?? 0;
+
+        // 下标减一
+        post_idx--;
+        // 构造右子树
+        root.right = helper(index + 1, in_right);
+        // 构造左子树
+        root.left = helper(in_left, index - 1);
+        return root;
+    }
+
+    // 从后序遍历的最后一个元素开始
+
+    // 建立（元素，下标）键值对的哈希表
+    // let idx = 0;
+    inorder.forEach((val, idx) => {
+        idx_map.set(val, idx);
+    });
+    return helper(0, inorder.length - 1);
+}