剑指 Offer 28. 对称的二叉树

方法一：递归

/**
 * 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) {
    return root == null ? true : recur(root.left, root.right);
};

const recur = (left, right) => {
    if(left == null && right == null) {
        return true;
    }
    if(left == null || right == null || left.val != right.val) {
        return false;
    }
    return recur(left.left, right.right) && recur(left.right, right.left);
}

方法二：栈

/**
 * 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;
    let stack1 = [], stack2 = [];
    let left = root.left, right = root.right;
    while(stack1.length != 0 || stack2.length != 0 || left || right) {
        while(left && right) {
            stack1.push(left);
            stack2.push(right);
            left = left.left;
            right = right.right;
        }
        if(left || right) {
            return false;
        }
        left = stack1.pop();
        right = stack2.pop();
        if(left.val != right.val) {
            return false;
        }
        left = left.right;
        right = right.left;
    }
    return true;
};