404. 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.

還是概念，左葉子節點，對於一個結點，其左子樹不爲空，且其左子樹的左子節點和右子節點都爲空，則其爲左葉子節點。

/**
 * 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 rs = 0;
        sum(root, rs);
        return rs;
    }
    
    void sum(TreeNode* root, int& val){
        if(!root){
            return;
        }    

        if(root->left && !root->left->left && !root->left->right){
            val += root->left->val;
        }
        sum(root->left, val);
        sum(root->right, val);
    }
    
};