【leetcode】Path Sum2

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum.

For example:
Given the below binary tree and sum = 22,

              5
             / 
            4   8
           /   / 
          11  13  4
         /      / 
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

 解体思路：利用递归深度优先遍历，保存中间结果。

/**
 * Definition for binary tree
 * 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> > result;
        vector<int> intermediate;
        dfs(root, sum, intermediate, result);
        return result;
    }
    
    void dfs(TreeNode *root, int gap, vector<int> &intermediate, 
            vector<vector<int> > &result){
        if(root == nullptr)
            return;
        intermediate.push_back(root->val);
        
        if(root->left == nullptr && root->right == nullptr){
            if(gap == root->val)
                result.push_back(intermediate);
        }
        
        dfs(root->left, gap - root->val, intermediate, result);
        dfs(root->right, gap - root->val, intermediate, result);
        
        intermediate.pop_back();//弹出回退
    }
};