113. 路径总和 II

给定一个二叉树和一个目标和，找到全部从根节点到叶子节点路径总和等于给定目标和的路径。
说明: 叶子节点是指没有子节点的节点。

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:
void preorder(TreeNode* node,int &path_val,vector<int> &path,vector<vector<int> > &result,int &sum)
{
    if(!node)
    return ;
    path_val += node->val;
    path.push_back(node->val);
    if(node->left==NULL && node->right == NULL && path_val == sum)
    {
        result.push_back(path);
    }
    preorder(node->left,path_val,path,result,sum);
    preorder(node->right,path_val,path,result,sum);
    path_val -= node->val;
    path.pop_back();
}
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        int path_val = 0;
        vector<vector<int> >result;
        vector<int> path;
        preorder(root,path_val,path,result,sum);
        return result;
    }
};