leetcode 110. Balanced Binary Tree（平衡二叉树的判断） DFS深度优先遍历

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

本题考查的是平衡二叉树的判断，平衡二叉树是一 棵空树或它的左右两个子树的高度差的绝对值不超过1，那么直接递归判断即可。

1） 递归判断二叉树是否是平衡二叉树

2） 递归找二叉树高度,求高度也是用递归

代码如下：

/*class TreeNode {      int val;      TreeNode left;      TreeNode right;      TreeNode(int x) { val = x; }}*/ /*  *   平衡二叉树：它是一 棵空树或它的左右两个子树的高度差的绝对值不超过1  *   1.递归判断二叉树是否是平衡二叉树  *   2.递归找二叉树高度,求高度也是用递归  *   递归停止条件：  *   递归到叶子节点时终止，root==null时，返回0。  */public class Solution{    public boolean isBalanced(TreeNode root)     {        if(root ==null)            return true;        int heightLeft=getHeight(root.left);        int heightRight=getHeight(root.right);        if(Math.abs(heightLeft-heightRight)<=1)            return isBalanced(root.left) && isBalanced(root.right);        else            return false;    }    int getHeight(TreeNode root)     {        if(root==null)            return 0;        else        {            int heightLeft=getHeight(root.left)+1;            int heightRight=getHeight(root.right)+1;            return Math.max(heightLeft, heightRight);        }    }}

下面是C++的做法，就是一个简单的DFS深度优先遍历的应用

代码如下：

#include <iostream>#include <vector>#include <queue>#include <algorithm>using namespace std;/*struct TreeNode{    int val;    TreeNode *left;    TreeNode *right;    TreeNode(int x) : val(x), left(NULL), right(NULL) {}};*/class Solution {public:    bool isBalanced(TreeNode* root)     {        if (root == NULL)            return true;        else        {            int left = getHeight(root->left) + 1;            int right = getHeight(root->right) + 1;            if (abs(left - right) <= 1)                return isBalanced(root->left) && isBalanced(root->right);            else                return false;        }    }    int getHeight(TreeNode* root)    {        if (root == NULL)            return 0;        else        {            int left = getHeight(root->left) + 1;            int right = getHeight(root->right) + 1;            return max(left, right);        }    }};