TZOJ数据结构实验：左叶子之和

int sumOfLeftLeaves(struct TreeNode* root)
{
        if (root == NULL)
        {
            return ;
        }//递归结束条件
        if (root->left && root->left->left == NULL && root->left->right == NULL)
            return root->left->val + sumOfLeftLeaves(root->right);
        return sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right);
    }