LeetCode : Sum of Left Leaves

Find the sum of all left leaves in a given binary tree.

Example:

3

/ \
9 20
/ \
15 7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

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