LeetCode-101. Symmetric Tree

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问题描述

Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

Constraints:

  • The number of nodes in the tree is in the range [1, 1000].
  • -100 <= Node.val <= 100

解答

方法一：递归
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/*
 * @Description: 101. Symmetric Tree
 * @Author: libk
 * @Github: https://github.com/libk
 */
/**
 * 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)
 * }
 */
/**
 * @description: 递归
 * @param {TreeNode} root
 * @return {boolean}
 */
const isSymmetric = function (root) {
  if (!root) {
    return true
  }

  const dfs = (leftNode, rightNode) => {
    if (!leftNode && !rightNode) {
      return true
    }
    if (!leftNode || !rightNode || leftNode.val !== rightNode.val) {
      return false
    }
    return dfs(leftNode.left, rightNode.right) && dfs(leftNode.right, rightNode.left)
  }
  return dfs(root.left, root.right)
}
方法二：迭代
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/*
 * @Description: 101. Symmetric Tree
 * @Author: libk
 * @Github: https://github.com/libk
 */
/**
 * 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)
 * }
 */
/**
 * @description: 迭代
 * @param {TreeNode} root
 * @return {boolean}
 */
const isSymmetric = function (root) {
  if (!root) {
    return true
  }
  let queue = []
  queue.push(root.left)
  queue.push(root.right)

  while (queue.length) {
    let leftNode = queue.shift()
    let rightNode = queue.shift()

    if (!leftNode && !rightNode) {
      continue
    }
    if (!leftNode || !rightNode || leftNode.val !== rightNode.val) {
      return false
    }
    
    queue.push(leftNode.left)
    queue.push(rightNode.right)

    queue.push(leftNode.right)
    queue.push(rightNode.left)
  }
  return true
}