404. Sum of Left Leaves

class Solution {
public:
    int sumOfLeftLeaves(TreeNode* root) {
        if (!root) return 0;
        if (root->left && !root->left->left && !root->left->right) return root->left->val + sumOfLeftLeaves(root->right);//注意要保证是左节点, 节点的左右子都应该为空
        return sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right);
    }
};

• https://discuss.leetcode.com/topic/60467/3-line-c-solution
• http://www.cnblogs.com/grandyang/p/5923559.html