LeetCode in Kotlin

77. Combinations

Medium

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

You may return the answer in any order.

Example 1:

Input: n = 4, k = 2

Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]

Explanation: There are 4 choose 2 = 6 total combinations. Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.

Example 2:

Input: n = 1, k = 1

Output: [[1]]

Explanation: There is 1 choose 1 = 1 total combination.

Constraints:

• 1 <= n <= 20
• 1 <= k <= n

Solution

class Solution {
    fun combine(n: Int, k: Int): List<List<Int>> {
        val ans: MutableList<List<Int>> = ArrayList()
        // Boundary case
        if (n > 20 || k < 1 || k > n) {
            return ans
        }
        backtrack(ans, n, k, 1, ArrayDeque())
        return ans
    }

    private fun backtrack(ans: MutableList<List<Int>>, n: Int, k: Int, s: Int, stack: ArrayDeque<Int>) {
        // Base case
        // If k becomes 0
        if (k == 0) {
            ans.add(ArrayList(stack))
            return
        }
        // Start with s till n-k+1
        for (i in s..n - k + 1) {
            stack.addLast(i)
            // Update start for recursion and decrease k by 1
            backtrack(ans, n, k - 1, i + 1, stack)
            stack.removeLast()
        }
    }
}

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