leetcode#404. 左葉子之和

遞歸求解，C++實作如下：

class Solution {
public:
    int sumOfLeftLeaves(TreeNode* root) 
    {
        if(root == NULL)
            return ;
        if(root->left != NULL && root->left->left == NULL && root->left->right == NULL)
            return root->left->val + sumOfLeftLeaves(root->right);
        return sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right);
    }
};