Problem Description
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:
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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.
Solution
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class Solution {
public List<List<Integer>> combine(int n, int k) {
return combineHelper(n, k, 1, new ArrayList<Integer>(), new ArrayList<List<Integer>>());
}
//recursive helper that generates all combinations.
public List<List<Integer>> combineHelper(int n, int k, int idx, List<Integer> currentList, List<List<Integer>> all){
// Base case: If the current combination size reaches 'k', add it to the result list 'all'.
if(currentList.size() == k){
all.add(new ArrayList<>(currentList)); //make sure to create a new array and all that to the complete list
return all;
}
// Iterate over possible numbers that can be included in the combination.
for(int i=idx; i<=n; i++){
// Add the current number to the combination.
currentList.add(i);
// Recursively generate combinations by moving to the next index and considering the updated currentList.
combineHelper(n, k, i+1, currentList, all);
// Backtrack: Remove the last added number to explore other possibilities.
currentList.remove(currentList.size()-1);
}
return all;
}
}