[LeetCode] 110. Balanced Binary Tree 解题思路

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.

问题：给定一个二叉树，判断其是否平衡。

当二叉树中所有节点的左右子树高度差不大于 1 时，这个二叉树视为平衡二叉树。

解题思路应该算一种分治思想。根据定义，递归地判断，实现判断。

    const int unbalanceKids = -1;

    /**
     * count the number of children of node and itself
     */ 
    int height(TreeNode* node){
        
        int heightL = 0;
        if (node->left != NULL){
            heightL = height(node->left);
        }
        
        if(heightL == unbalanceKids){
            return unbalanceKids;
        }
        
        int heightR = 0;
        if (node->right != NULL){
            heightR = height(node->right);
        }
        
        if(heightR == unbalanceKids){
            return unbalanceKids;
        }
        
        if (abs(heightL - heightR) > 1){
            return unbalanceKids;
        }
        
        return max(heightL, heightR) + 1;
    }

    bool isBalanced(TreeNode* root) {
        
        if (root == NULL){
            return true;
        }
        
        int res  = height(root);
        if (res == unbalanceKids){
            return false;
        }
        
        return true;
    }