110. Balanced Binary Tree. (Easy)

Given a binary tree, determine if it is .

 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: true

Example 2:

Input: root = [1,2,2,3,3,null,null,4,4]
Output: false

Example 3:

Input: root = []
Output: true

 

Constraints:

• The number of nodes in the tree is in the range [0, 5000].
• -104 <= Node.val <= 104

A:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
 * right(right) {}
 * };
 */
class Solution {
public:
    bool isBalanced(TreeNode* root) {
        int depth = 0;
        return helper(root, depth);
    }
private:
    bool helper(TreeNode* root, int& depth) {
        if (!root) {
            depth = 0;
            return true;
        }
        int dL = 0, dR = 0;
        if (root->left) {
            auto resL = helper(root->left, dL);
            if (!resL) {
                return false;
            }
        }
        if (root->right) {
            auto resR = helper(root->right, dR);
            if (!resR) {
                return false;
            }
        }
        depth = 1 + max(dL, dR);
        return abs(dL - dR) <= 1;
    }
};

Mistakes: