Balanced Binary Tree——数是否是平衡，即任意节点左右字数高度差不超过1

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.

/**
 * Definition for binary tree
 * 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;
        int l=height(root->left);
        int r=height(root->right);
        if(l<0||r<0||l-r>1||r-l>1) return false;
        return true;
    }
    int height(TreeNode *root){
        if(root==NULL) return 0;
        int l=height(root->left);
        int r=height(root->right);
        if(l<0||r<0||l-r>1||r-l>1) return -1;
        return max(l,r)+1;
    }
};

其他做法：

转自：http://blog.csdn.net/feliciafay/article/details/18348065

// 104ms过大集合  
/** 
 * Definition for binary tree 
 * struct TreeNode { 
 *     int val; 
 *     TreeNode *left; 
 *     TreeNode *right; 
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 
 * }; 
 */  
class Solution {  
public:  
    int height(TreeNode* root) {  
        if(root==NULL)  
            return 0;  
        else{  
            int l=height(root->left);  
            int r=height(root->right);  
            return 1+((l>r)?l:r);  
        }  
    }  
    bool isBalanced(TreeNode *root) {  
        if(root==NULL)  
            return true;  
        else{  
            int l,r;  
            l=height(root->left);  
            r=height(root->right);  
            if((l>r+1)||(r>l+1))  
                return false;  
            else  
                return isBalanced(root->left)&&isBalanced(root->right);  
        }  
    }  
};  

// 68ms过大集合  简洁版
public:  
int cntHeight(TreeNode *root) {  
    if(root == NULL) return 0;  
    int l = cntHeight(root->left);  
    int r = cntHeight(root->right);  
    if(l < 0 || r < 0 || abs(l-r) > 1) return -1; //自定义 return -1，表示不平衡的情况  
    else  return max(l, r) + 1;  
}  
bool isBalanced(TreeNode *root) {  
    if(root == NULL) return true;      
    int l = cntHeight(root->left);  
    int r = cntHeight(root->right);  
    if(l < 0 || r < 0 || abs(l-r) > 1) return false;  
    else return true;  
}  

// 68ms过大集合  更简洁版
public:  
int cntHeight(TreeNode *root) {  
    if(root == NULL) return 0;  
    int l = cntHeight(root->left);  
    int r = cntHeight(root->right);  
    if(l < 0 || r < 0 || abs(l-r) > 1) return -1; //自定义 return -1，表示不平衡的情况  
    else  return max(l, r) + 1;  
}  
bool isBalanced(TreeNode *root) {  
    if(root == NULL) return true;      
    int l = cntHeight(root->left);  
    int r = cntHeight(root->right);  
    if(l < 0 || r < 0 || abs(l-r) > 1) return false;  
    else return true;  
}