101.对称二叉树.js

/*
 * @lc app=leetcode.cn id=101 lang=javascript
 *
 * [101] 对称二叉树
 *
 * https://leetcode-cn.com/problems/symmetric-tree/description/
 *
 * algorithms
 * Easy (53.13%)
 * Likes:    1423
 * Dislikes: 0
 * Total Accepted:    345K
 * Total Submissions: 623.5K
 * Testcase Example:  '[1,2,2,3,4,4,3]'
 *
 * 给定一个二叉树，检查它是否是镜像对称的。
 * 
 * 
 * 
 * 例如，二叉树 [1,2,2,3,4,4,3] 是对称的。
 * 
 * ⁠   1
 * ⁠  / \
 * ⁠ 2   2
 * ⁠/ \ / \
 * 3  4 4  3
 * 
 * 
 * 
 * 
 * 但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:
 * 
 * ⁠   1
 * ⁠  / \
 * ⁠ 2   2
 * ⁠  \   \
 * ⁠  3    3
 * 
 * 
 * 
 * 
 * 进阶：
 * 
 * 你可以运用递归和迭代两种方法解决这个问题吗？
 * 
 */

// @lc code=start
/**
 * 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)
 * }
 */
/**
 * 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) {
  if(!root) return true
  return isEuqal(root.left, root.right)
};

const isEuqal = (left, right)=>{
  if(left===null&&right===null) return true
  if(left===null||right===null) return false
  if(left.val!==right.val) return false
  // 递归判断对称
  return isEuqal(left.left, right.right) && isEuqal(left.right, right.left)
}
// @lc code=end