101. Symmetric Tree

题目

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3

Note:

Bonus points if you could solve it both recursively and iteratively.

题目大意

这一题要求判断 2 颗树是否是左右对称的。

解题思路

• 这道题是几道题的综合题。将根节点的左字数反转二叉树，然后再和根节点的右节点进行比较，是否完全相等。
• 反转二叉树是第 226 题。判断 2 颗树是否完全相等是第 100 题。

代码

package leetcode

import (
	"github.com/halfrost/leetcode-go/structures"
)

// TreeNode define
type TreeNode = structures.TreeNode

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */

// 解法一 dfs
func isSymmetric(root *TreeNode) bool {
	if root == nil {
		return true
	}
	return isMirror(root.Left, root.Right)
}

func isMirror(left *TreeNode, right *TreeNode) bool {
	if left == nil && right == nil {
		return true
	}
	if left == nil || right == nil {
		return false
	}
	return (left.Val == right.Val) && isMirror(left.Left, right.Right) && isMirror(left.Right, right.Left)
}

// 解法二
func isSymmetric1(root *TreeNode) bool {
	if root == nil {
		return true
	}
	return isSameTree(invertTree(root.Left), root.Right)
}

func isSameTree(p *TreeNode, q *TreeNode) bool {
	if p == nil && q == nil {
		return true
	} else if p != nil && q != nil {
		if p.Val != q.Val {
			return false
		}
		return isSameTree(p.Left, q.Left) && isSameTree(p.Right, q.Right)
	} else {
		return false
	}
}

func invertTree(root *TreeNode) *TreeNode {
	if root == nil {
		return nil
	}
	invertTree(root.Left)
	invertTree(root.Right)
	root.Left, root.Right = root.Right, root.Left
	return root
}

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