Leetcode 563. Binary Tree Tilt

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int findTilt(TreeNode* root) {
        if(!root) {
            return 0;
        }
        int tilt = 0;
        dfs(root, tilt);
        return tilt;
    }
    
private:
    int dfs(TreeNode* root, int& tilt) {
        if(!root) {
            return 0;
        }
        int leftSum = dfs(root->left, tilt);
        int rightSum = dfs(root->right, tilt);
        tilt += abs(leftSum - rightSum);
        return root->val + leftSum + rightSum;
    }
};