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;
     }
 }