g0001_0100.s0077_combinations.Solution.kt Maven / Gradle / Ivy
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Kotlin-based LeetCode algorithm problem solutions, regularly updated
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()
}
}
}