struct TreeNode {

    int val;

    TreeNode *left;

    TreeNode *right;

    TreeNode(int x) : val(x), left(NULL), right(NULL) {}

};

class Solution {

public:

    bool hasPathSum(TreeNode *root, int sum) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        if(!root) return false;

        flag = false;

        target = sum;

        preOrder(root,);

        return flag;

    }

    void preOrder(TreeNode* node,int sum){

        sum = node->val + sum; //递归前，加上当前节点

        if(node->left)

        {

            preOrder(node->left,sum);

        }

        if(node->right)

        {

            preOrder(node->right,sum);

        }

        if(!node->left && !node->right && sum == target) //递归结束条件：到了叶子节点

        {

            flag = true;

        }

    }

private:

    bool flag;

    int target;

};