110. Balanced Binary Tree

1. 问题描述

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.
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Tags: Tree, Depth-first Search
Similar Problems: (E) Maximum Depth of Binary Tree

2. 解题思路

• 先求左右子树的深度，再递归判断

3. 代码

class Solution {
public:
    bool isBalanced(TreeNode* root) 
    {
        if (NULL == root)
        {
            return true;
        }
        int ld = getBinaryDepth(root->left);
        int rd = getBinaryDepth(root->right);
        if (ld-rd>1 || rd-ld>1)
        {
            return false;
        }
        else
        {
            return isBalanced(root->left) && isBalanced(root->right);
        }
    }
private:
    int getBinaryDepth(TreeNode* root)
    {
        if (NULL == root)
        {
            return 0;
        }
        int ld = 1 + getBinaryDepth(root->left);
        int rd = 1 + getBinaryDepth(root->right);
        return ld > rd ? ld : rd;
    }
};

4. 反思

• 在CSDN上发现一种做法，可以把时间复杂度降低为O(N)
• http://blog.csdn.net/feliciafay/article/details/18348065