LeetCode113 Path Sum II

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

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]
]

分析：

回溯法处理即可，利用helper函数遍历所有路径，达到sum时添加到result里。

代码：

/**
 * 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 {
    vector<vector<int>> result;
    void helper(TreeNode* root, int sum, vector<int>& vec) {
        if (root == nullptr) {
            return;
        }
        vec.push_back(root -> val);
        sum -= root -> val;
        if (root -> left == nullptr && root -> right == nullptr) {
            if (sum == 0) {
                result.push_back(vec);
            }
            else {
                vec.pop_back();
                sum += root -> val;
                return;
            }
        }
        if (root -> left != nullptr) {
            helper(root -> left, sum, vec);
        }
        if (root -> right != nullptr) {
            helper(root -> right, sum, vec);
        }
        vec.pop_back();
        sum -= root -> val;
    }
public:
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        vector<int> vec;
        helper(root, sum, vec);
        return result;
    }
};