Day 4, leetcode 104, 110 , Maximum Depth of Binary Tree, Balanced Binary Tree

Maximum Depth of Binary Tree

/**
 * 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 maxDepth(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (root == NULL) return 0;
        int left = maxDepth(root->left) + 1;
        int right = maxDepth(root->right) + 1;    
        if (left > right) return left;
        else return right;
        
        
    }
};

Balanced Binary Tree
---------------------------------------- 
Efficiency: O(n^2), is there a better way to do this?

/**
 * 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 depth(TreeNode *root) {
        if (root == NULL) return 0;
        int left = depth(root->left) + 1;
        int right = depth(root->right) + 1;   
        if (left > right) return left;
        else return right;
    }
    
    bool isBalanced(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (root == NULL) return true;
        return isBalanced(root->left) && isBalanced(root->right) && abs(depth(root->left) - depth(root->right)) < 2;
    }
};

Update on Jan-18-2015
O(n), each node is only visited once

/**
 * 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 flag;
    int getHeight(TreeNode *root) {
        if (root == NULL) {
            return 0;
        }
        
        int leftH = getHeight(root->left) + 1;
        int rightH = getHeight(root->right) + 1;
        if (abs(leftH - rightH) > 1) flag = false;
        return max(leftH,rightH);
    }

    bool isBalanced(TreeNode *root) {
        flag = true;
        getHeight(root);
        
        return flag;
        
    }
};

Update on Feb-07-2019, in Java
Get the height of each node

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isBalanced(TreeNode root) {
        return dfs(root) != -1;
    }
    
    private int dfs(TreeNode root) {
        if (root == null) return 0;
        int left = dfs(root.left);
        int right = dfs(root.right);
        if (left == -1 || right == -1) return -1;
        if (Math.abs(left - right) > 1) return -1;
        return Math.max(left, right) + 1;
    }
}