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 falseconst 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;
};