二叉树 - 对称二叉树

101. 对称二叉树

方法一：递归判断

/**
 * 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) {
    // 使用递归遍历左右子树 递归三部曲
    // 1. 确定递归的参数 root.left root.right 和返回值true false
    const compareNode = function(left, right) {
        // 2. 确定终止条件 空的情况
        if (left == null && right !== null || left !== null && right === null) {
            return false;
        } else if (left === null && right === null) {
            return true;
        } else if (left.val !== right.val) {
            return false;
        }
        // 3. 确定单层递归逻辑
        let outSide = compareNode(left.left, right.right);
        let inSide = compareNode(left.right, right.left);
        return outSide && inSide;
    }
    if (root === null) {
        return true;
    }
    return compareNode(root.left, root.right);
};

方法二：队列实现迭代判断

/**
 * 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) {
    // 迭代方法判断是否是对称二叉树
    // 首先判断root是否为空
    if (root === null) {
        return true;
    }
    let queue = [];
    queue.push(root.left);
    queue.push(root.right);
    while (queue.length) {
        let leftNode = queue.shift();  // 左节点
        let rightNode = queue.shift();  // 右节点
        if (leftNode === null && rightNode === null) {
            continue;
        }
        if (leftNode === null || rightNode === null || leftNode.val !== rightNode.val) {
            return false;
        }
        queue.push(leftNode.left);  // 左节点左孩子入队
        queue.push(rightNode.right);  // 右节点右孩子入队
        queue.push(leftNode.right);  // 左节点右孩子入队
        queue.push(rightNode.left);  // 右节点左孩子入队
    }
    return true;
};

方法三：栈实现迭代判断

/**
 * 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) {
    // 迭代方法判断是否是对称二叉树
    // 首先判断root是否为空
    if (root === null) {
        return true;
    }
    let stack = [];
    stack.push(root.left);
    stack.push(root.right);
    while (stack.length) {
        let rightNode = stack.pop();  // 左节点
        let leftNode = stack.pop();  // 右节点
        if (leftNode === null && rightNode === null) {
            continue;
        }
        if (leftNode === null || rightNode === null || leftNode.val !== rightNode.val) {
            return false;
        }
        stack.push(leftNode.left);  // 左节点左孩子入栈
        stack.push(rightNode.right);  // 右节点右孩子入栈
        stack.push(leftNode.right);  // 左节点右孩子入栈
        stack.push(rightNode.left);  // 右节点左孩子入栈
    }
    return true;
};