25.对称二叉树

题目描述：

代码实现：

• 递归法，遍历树，判断左右节点是否相等即可。
• 时间复杂度：O(n)

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isSymmetric = function(root) {
    if (root === null) return true
    return issymmetric(root.left, root.right)
};

var  issymmetric = (left, right) => {
    if (left === null && right === null) {
        return true
    }
    if (left === null || right === null) {
        return false
    }
    return left.val === right.val && issymmetric(left.left, right.right) && issymmetric(left.right, right.left)
}