77. Combinations (Medium)

https://leetcode.com/problems/combinations/

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

Example:

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

Solutions

1.

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<Integer> track = new ArrayList<>();

        List<List<Integer>> ans = new ArrayList<>();

        getNext(0, n, k, track, ans);

        return ans;
    }

    private void getNext(int depth, int n, int k, List<Integer> track, List<List<Integer>> ans) {
        if (k == track.size()) {
            // create a new copy, do not refer to it
            List<Integer> solution = new ArrayList<>();
            for (int x : track) {
                solution.add(x);
            }

            ans.add(solution);

            return;
        }

        for (int j = depth + 1; j <= n; j++) {
            track.add(j);
            getNext(j, n, k, track, ans);
            track.remove(track.size()-1);
        }
    }
}

2.

class Solution2 {
    public List<List<Integer>> combine(int n, int k) {
        List<Integer> track = new ArrayList<>();

        List<List<Integer>> ans = new ArrayList<>();

        getNext(1, n, k, track, ans);

        return ans;
    }

    private void getNext(int depth, int n, int k, List<Integer> track, List<List<Integer>> ans) {
        if (track.size() == k) {
            // create a new copy, do not refer to it
            List<Integer> sol = new ArrayList<>(track);

            ans.add(sol);

            return;
        }

        // above code should not be place at the head of function, otherwise mistakes occurs
        if (depth > n) {
            return;
        }

        track.add(depth);
        getNext(depth + 1, n, k, track, ans);
        track.remove(track.size() - 1);

        getNext(depth + 1, n, k, track, ans);
    }
}

Incorrect Solutions

References

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