563. 二叉树的坡度

C代码：

int sum;

int dfs(struct TreeNode* root) {
    if (root == NULL) {
        return 0;
    }
    int left = dfs(root->left);
    int right = dfs(root->right);
    sum += fabs(left - right);
    return root->val + left + right;
}

int findTilt(struct TreeNode* root){
    sum = 0;
    dfs(root);
    return sum;
}