​LeetCode刷题实战404：左叶子之和

class Solution {
public:

    void helper(TreeNode* root,int& sum,bool is_left){
        if(root==NULL){//空节点返回
            return;
        }
        //为叶子结点
        if(root->left==NULL&&root->right==NULL){
            if(is_left){//为左结点
                sum+=root->val;
            }
            return;
        }
        helper(root->left,sum,true);//遍历左子树，标识是左子树
        helper(root->right,sum,false);//遍历右子树，标识不是左子树
    }

    int sumOfLeftLeaves(TreeNode* root) {
      //处理特殊的情形
        if(root==NULL||(root->left==NULL&&root->right==NULL)){
            return 0;
        }
        int sum=0;
        helper(root,sum,true);
        return sum;
    }
};