Balanced Binary Tree （二叉树DFS）

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.

判断一棵树是否为平衡二叉树。

思路：典型的递归，从第一个根节点开始，看左子树和右子树的深度差值是否小于等于1，然后再看由根节点的左孩子和右孩子构成的两课树是否都符合条件。

代码：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    int maxdepth;
public:
    void solve(TreeNode *root,int depth){
        if(root==NULL)
            return;
        if(depth>maxdepth)
            maxdepth=depth;
        solve(root->left,depth+1);
        solve(root->right,depth+1);
    }
    int getDepth(TreeNode *root) {
        maxdepth=0;
        solve(root,1);
        return maxdepth;
    }
    bool isBalanced(TreeNode *root) {
        if(root==NULL) 
            return true;
        int l,r;
        if(root->left!=NULL){
            l=getDepth(root->left);
        }else{
            l=0;
        }
        if(root->right!=NULL){
            r=getDepth(root->right);
        }else{
            r=0;
        }
        return abs(l-r)<=1&&isBalanced(root->left)&&isBalanced(root->right);
    }
};