404. Sum of Left Leaves

Well, very straightforward solution.

1:  /**  
2:   * Definition for a binary tree node.  
3:   * struct TreeNode {  
4:   *   int val;  
5:   *   TreeNode *left;  
6:   *   TreeNode *right;  
7:   *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
8:   * };  
9:   */  
10:  class Solution {  
11:  public:  
12:    int sumOfLeftLeaves(TreeNode* root) {  
13:      if (root == NULL) return 0;  
14:      int sum = 0;  
15:      if (isLeaf(root->left)) sum += root->left->val + sumOfLeftLeaves(root->right);  
16:      else sum = sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right);  
17:      return sum;  
18:    }  
19:    bool isLeaf(TreeNode *root) {  
20:      if (root == NULL) return false;  
21:      if (root->left == NULL && root->right == NULL) return true;  
22:      return false;  
23:    }  
24:  };