LeetCode-404. Sum of Left Leaves

问题：https://leetcode.com/problems/sum-of-left-leaves/?tab=Description
Find the sum of all left leaves in a given binary tree.求左叶子节点的和。
Example:
3
/ \
9 20
/ \
15 7
There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
分析：递归，逐个节点查找，如果有左节点就加上它的值。
参考C++代码：

/** * 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 sumOfLeftLeaves(TreeNode* root) {        if(root==NULL) return 0;        if(root->left !=NULL && root->left->left==NULL && root->left->right==NULL)            return root->left->val+sumOfLeftLeaves(root->right);        return sumOfLeftLeaves(root->right)+sumOfLeftLeaves(root->left);    }};