和为定值的m个数

和为定值的m个数

np 问题（时间复杂度o(2**n)）

code1:

暴力递归

解决 只有正数的情况

class Solution {
    private List<List<Integer>> res = new ArrayList<List<Integer>>();
    public List<List<Integer>> fourSum(int[] nums, int target) {
        boolean[] a = new boolean[nums.length];
        for(int i = 0;i <a.length-1;i++)
            a[i] = false;
        sum(nums,a,0,0,target);
        return res;
    }
    private void sum(int[] nums,boolean[] a,int i,int has,int target){
        if(i>a.length-1) return;
        if(has + nums[i] == target){
            a[i] = true;
            
            res.add(add_res(nums,a));
            a[i] = false;
        }
        a[i] = true;
        sum(nums,a,i+1,has+nums[i],target);
        a[i] = false;
        sum(nums,a,i+1,has,target);
    }
    private List<Integer> add_res (int[] nums,boolean[] a){
        List<Integer> res = new ArrayList<Integer>();
        for(int i = 0;i<a.length;i++)
            if(a[i])
                res.add(nums[i]);
        return res;
    }
}