77. 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],

回溯法，并且规定顺序。

boolean[] onstack;List<List<Integer>> retlist;public List<List<Integer>> combine(int n, int k){onstack=new boolean[n];ArrayList<Integer> arraylist=new ArrayList<>(100);long factor_n=1;long factor_k=1;for(int i=n,j=0;j<k;i--,j++)factor_n*=i;for(int i=2;i<=k;i++)factor_k*=i;long capacity=factor_n/factor_k;retlist=new ArrayList<>((int)capacity);dfs(n, k, 0, arraylist);return retlist;}public void dfs(int n,int k,int cur,ArrayList<Integer> arraylist){if(cur==k){retlist.add(new ArrayList<>(arraylist));return ;}for(int i=1;i<=n;i++){if((arraylist.isEmpty())||(!arraylist.isEmpty()&&i>arraylist.get(arraylist.size()-1)))if(!onstack[i-1]){onstack[i-1]=true;arraylist.add(i);dfs(n, k, cur+1, arraylist);arraylist.remove(arraylist.size()-1);onstack[i-1]=false;}}}