110. Balanced Binary Tree

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 a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int depth(TreeNode* r){
        return r == NULL ? 0:(1+ max(depth(r->left), depth(r->right)));
    }

    bool isBalanced(TreeNode* root) {
        if(root == NULL)
            return true;

        int lh = depth(root->left);
        int rh = depth(root->right);

        return abs(lh-rh)<=1 && isBalanced(root->left) && isBalanced(root->right);
    }
};

下面只需要遍历所有节点一次

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isBalanced(TreeNode* root) {
        int d = 0;
        return isBalanced(root, d);

    }
private:
    bool isBalanced(TreeNode* r, int &depth){
        if(r == NULL){
            depth = 0;
            return true;
        }

        int lh, rh;
        if(isBalanced(r->left, lh) && isBalanced(r->right, rh)){
            if(abs(lh - rh) <= 1){
                depth = max(lh, rh) + 1;
                return true;
            }
        }
        return false;
    }
};