coding-Interview/Combinations.java at master · arjunmullick/coding-Interview · GitHub

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package com.leetcode;

import java.util.ArrayList;

import java.util.List;

public class Combinations {

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

/**

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.

Time complexity: O(2^n)

Space complexity: O(n)

**/

class Solution {

public List<List<Integer>> combine(int n, int k) {

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

backtrack(1,n,k,new ArrayList<>(),result);

return result;

}

public void backtrack(int start, int n , int k, List<Integer> list, List<List<Integer>> result){

if(list.size() == k){

result.add(new ArrayList<>(list));

}

for(int i= start ; i <=n ; i++){

list.add(i);

backtrack(i+1,n,k,list,result);

list.remove(list.size()-1);

}

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