77. Combinations

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

You may return the answer in any order.

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Example 1:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

Example 2:

Input: n = 1, k = 1
Output: [[1]]

Constraints:

1 <= n <= 20
1 <= k <= n

Solution

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class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> current = new ArrayList<>();
        back(1, n, k, current, res);
        return res;
    }
    void back(int value, int n, int k,
              List<Integer> current, 
              List<List<Integer>> res) {
        if (current.size() == k) {
            res.add(new ArrayList<>(current));
            return;
        }
        for (int i = value; i <= n; i++) {
            current.add(i);
            back(i + 1, n, k, current, res);
            current.remove(current.size() - 1);
        }
    }
}

Solution#

Solution