[leetcode_77] Combinations

A simple search problem, though the problem statement wasn’t very clear (or maybe my English isn’t great). Basically, given n, the universal set is 1, 2, 3…n. Choose k elements to form subsets, and enumerate all such subsets.

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class Solution {
public:
    void combineStep(int n, int step, int k, vector<int>& item, vector<vector<int>>& ans, int count)
    {
        if(step > n+1)
            return;
        if(count == k)
        {
            ans.push_back(item);
            return ;
        }
        else
        {
            item.push_back(step);
            combineStep(n, step+1, k, item, ans, count+1);
            item.pop_back();
            combineStep(n, step+1, k, item, ans, count);
        }
    }
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> ans;
        ans.clear();
        vector<int> item;
        item.clear();
        combineStep(n, 1, k, item, ans, 0);
        return ans;
    }
};