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Leetcode-Combinations
Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
For example,
If n = 4 and k = 2, a solution is:
[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4],]
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Analysis:
We need consider the boundary and the return condition very carefully!
Solution:
1 public class Solution { 2 public List<List<Integer>> combine(int n, int k) { 3 List<List<Integer>> resSet = new ArrayList<List<Integer>>(); 4 if (n==0 || k==0) return resSet; 5 List<Integer> curList = new ArrayList<Integer>(); 6 combineRecur(resSet,curList,1,k,n,k); 7 return resSet; 8 } 9 10 public void combineRecur(List<List<Integer>> resSet, List<Integer> curList, int curIndex, int numLeft, int n, int k){11 if (numLeft==1){12 for (int i=curIndex;i<=n;i++){13 curList.add(i);14 List<Integer> temp = new ArrayList<Integer>();15 temp.addAll(curList);16 resSet.add(temp);17 curList.remove(curList.size()-1);18 }19 return;20 }21 22 23 for (int i=curIndex;i<=n-numLeft+1;i++){24 curList.add(i);25 combineRecur(resSet,curList,i+1,numLeft-1,n,k);26 curList.remove(curList.size()-1);27 }28 }29 30 }
Leetcode-Combinations
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