LeetCode OJ 77. Combinations

题目

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

解答

用递归，对i-n求k个数的组合，先确定一个数j（其中i <= j <= n + 1 - k），然后对(i + 1)-n求(k - 1)个数的组合，递归终止的条件是在一个数列中求1个数的组合，也就是将这个数列枚举一遍。

下面是AC的代码：

class Solution {
public:
    vector<vector<int>> generate_combinations(int start, int n, int k){
        vector<vector<int>> ret, temp;
        if(k == 1){
            for(int i = start; i <= n; i++){
                vector<int> combination;
                combination.push_back(i);
                ret.push_back(combination);
            }
            return ret;
        }
        for(int i = start; i + k <= n + 1; i++){
            temp = generate_combinations(i + 1, n, k - 1);
            for(vector<vector<int>>::iterator iter = temp.begin(); iter != temp.end(); iter++){
                iter->insert(iter->begin(), i);
                ret.push_back(*iter);
            }
        }
        return ret;
    }
    vector<vector<int>> combine(int n, int k) {
        return generate_combinations(1, n, k);
    }
};

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