数字组合III

组给出两个整数n和k,返回从1......n中选出的k个数的组合。

样例

例如 n = 4 且 k = 2

返回的解为:

[[2,4],[3,4],[2,3],[1,2],[1,3],[1,4]] 

解题

数字组合I  数组组合II

同样式DFS

本题只需要增加判断size 是否等于k的情况

public class Solution {
    /**
     * @param n: Given the range of numbers
     * @param k: Given the numbers of combinations
     * @return: All the combinations of k numbers out of 1..n
     */
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        // write your code here
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(n<0 || k<0)
            return result;
        int start = 1;
        ArrayList<Integer> list = new ArrayList<Integer>();
        DFS(n,k,start,result,list);
        return result;
    }
    public void DFS(int n,int k,int start ,ArrayList<ArrayList<Integer>> result,ArrayList<Integer> list){
        if(list.size() == k){
            ArrayList<Integer> tmp = new ArrayList<Integer>(list);
            if(!result.contains(tmp)) // 可不要
                result.add(tmp);
            return;
        }
        for(int i = start;i<=n ;i++){
            list.add(i);
            DFS(n,k,i+1,result,list);
            list.remove(list.size() -1);
        }
    }
}