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.

/**
 * 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) return 0;
        int sum = 0;
        dfs(root,sum,false);
        return sum;
    }
    void dfs(TreeNode *root,int &sum,bool isLeft)
    {   
        if(!root) return;
        if(root->left==NULL && root->right==NULL&&isLeft)
            sum+=root->val;
        dfs(root->left,sum,true);//借助变量的做法。
        dfs(root->right,sum,false); 
    }
};