101. Symmetric Tree

Leetcode link

题目简介

/**
 * @param {TreeNode} root
 * @return {boolean}
 */

题目给我们一个二叉树的根节点 root，要求我们判断该二叉树是否对称

解题思路

我们可以把 root 拆分成两个子树 left 与 right，这两者对称的条件是：

1. 两者同时为 null
2. left.val === right.val
3. left.right 与 right.left 对称
4. left.left 与 right.right 对称

必须同时满足这四个条件

Javascript

/**
 * 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
 * @return {boolean}
 */
var isSymmetric = function(root) {
    const isMirror = (left, right) => {
        if(!left && !right) {
            // if both left and right are null
            return true
        }
        if(!left || !right) {
            // if only 1 is null
            return false
        }

        return left.val === right.val && isMirror(left.right, right.left) && isMirror(left.left, right.right)

    }

    return isMirror(root.left, root.right)
};