balanced binary tree.

balanced_binary_tree.cpp

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 * 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. 


 */
class Solution {
public:
    bool isBalanced(TreeNode *root) {
        if (!root)
            return true;
        //get heights of left and right trees (height==-1 if already not balanced) 
        //        Note: this is needed to check "every node" has balanced subtree
        //    if abs(diff)  <= 1, true
        int leftH = getHeight(root->left);
        int rightH = getHeight(root->right);
        
        return leftH>=0 && rightH>=0 && abs(leftH - rightH) <= 1;
    }
    int getHeight(TreeNode *root) {
        if (!root)
            return 0;
        int leftH = getHeight(root->left);
        int rightH = getHeight(root->right);
        if (leftH < 0 || rightH < 0 )   //already not balanced
            return -1;
        if  (abs(leftH  - rightH) > 1)
            return -1;
        return 1 + max(leftH, rightH);
    }
};