【LeetCode热题100（39/100）】对称二叉树

题目地址： 链接
思路： 使用递归深度优先搜索（DFS）同时遍历左右子树，确保左右子树的结构和节点值对称。

/**
 * 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) {
    let ans = false;
    if(!root) return ans;
    const dfs = (left, right) => {
        if(left == null || right == null) return left == right;
        if(left.val != right.val) return false;
        return dfs(left.left, right.right) && dfs(left.right, right.left);
    }
    ans = dfs(root.left, root.right)
    return ans;
};