g0001_0100.s0077_combinations.Solution.kt Maven / Gradle / Ivy

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Kotlin-based LeetCode algorithm problem solutions, regularly updated

There is a newer version: 1.28

package g0001_0100.s0077_combinations

// #Medium #Backtracking #Algorithm_I_Day_11_Recursion_Backtracking
// #2023_07_10_Time_232_ms_(100.00%)_Space_41.1_MB_(95.65%)

class Solution {
    fun combine(n: Int, k: Int): List> {
        val ans: MutableList> = 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>, n: Int, k: Int, s: Int, stack: ArrayDeque) {
        // 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()
        }
    }
}