路径之和2

深度搜索从根结点到叶节点的路径，检查 各路径上所有结点的值的和是否为sum

利用vector模拟栈来存储路径

/**
 * 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:
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        vector<vector<int> > res;
        vector<int> path;//保存路径
        int path_value = 0;//当前路径的和
        preorder(root,sum,path_value,path,res);
        return res;
    }
    
    void preorder(TreeNode* node, int& sum,int& path_value,vector<int>& path, vector<vector<int> >& res)
    {
        if(!node)
            return;
        
        path_value += node->val;//当前路径的和加上当前这个结点的值
        path.push_back(node->val);//把当前结点加入路径中
        if( node->left==nullptr && node->right == nullptr && path_value == sum )
        {
            res.push_back(path);          
        }
        
        preorder(node->left,sum,path_value,path,res);
        preorder(node->right,sum,path_value,path,res);
        //当这个当前的结点的左右子树遍历完后，弹出它
        path_value -= node->val;
        path.pop_back();
        
    }
};