题目描述：

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.

代码实现：

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