Update 0106.从中序与后序遍历序列构造二叉树.md

优化js版本代码并添加注释说明

Author: jwhx

problems/0106.从中序与后序遍历序列构造二叉树.md

@@ -792,30 +792,27 @@ func findRootIndex(target int,inorder []int) int{
 
 ```javascript
 var buildTree = function(inorder, postorder) {
-    if (!postorder.length) return null
-    
-    let root = new TreeNode(postorder[postorder.length - 1])
-    
-    let index = inorder.findIndex(number => number === root.val)
-    
-    root.left = buildTree(inorder.slice(0, index), postorder.slice(0, index))
-    root.right = buildTree(inorder.slice(index + 1, inorder.length), postorder.slice(index, postorder.length - 1))
-    
-    return root
+    if (!preorder.length) return null;
+    const rootVal = postorder.pop(); // 从后序遍历的数组中获取中间节点的值， 即数组最后一个值
+    let rootIndex = inorder.indexOf(rootVal); // 获取中间节点在中序遍历中的下标
+    const root = new TreeNode(rootVal); // 创建中间节点
+    root.left = buildTree(inorder.slice(0, rootIndex), postorder.slice(0, rootIndex)); // 创建左节点
+    root.right = buildTree(inorder.slice(rootIndex + 1), postorder.slice(rootIndex)); // 创建右节点
+    return root;
 };
 ```
 
 从前序与中序遍历序列构造二叉树
 
 ```javascript
 var buildTree = function(preorder, inorder) {
-    if(!preorder.length)
-        return null;
-    let root = new TreeNode(preorder[0]);
-    let mid = inorder.findIndex((number) => number === root.val);
-    root.left = buildTree(preorder.slice(1, mid + 1), inorder.slice(0, mid));
-    root.right = buildTree(preorder.slice(mid + 1, preorder.length), inorder.slice(mid + 1, inorder.length));
-    return root;
+  if (!preorder.length) return null;
+  const rootVal = preorder.shift(); // 从前序遍历的数组中获取中间节点的值， 即数组第一个值
+  const index = inorder.indexOf(rootVal); // 获取中间节点在中序遍历中的下标
+  const root = new TreeNode(rootVal); // 创建中间节点
+  root.left = buildTree(preorder.slice(0, index), inorder.slice(0, index)); // 创建左节点
+  root.right = buildTree(preorder.slice(index), inorder.slice(index + 1)); // 创建右节点
+  return root;
 };
 ```