18. 4Sum

和3sum 一样,多了个循环

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        Arrays.sort(nums);
        List<List<Integer>> res=new ArrayList<List<Integer>>();
        for(int i=0;i<nums.length-3;i++){
            if(i>0&&nums[i]==nums[i-1]){
                continue;
            }
            for(int j=i+1;j<nums.length-2;j++){
                if(j>i+1&&nums[j]==nums[j-1]){
                    continue;
                }
                int k=j+1;
                int m=nums.length-1;
                while(k<m){
                    int sum = nums[i]+nums[j]+nums[k]+nums[m];
                    if(sum==target){
                        res.add(Arrays.asList(nums[i],nums[j],nums[k],nums[m]));
                        while(m-1>=0&&nums[m]==nums[--m]);
                        while(k+1<nums.length&&nums[k]==nums[++k]);
                    }else if(sum>target){
                        m--;
                    }else{
                        k++;
                    }
                }
            }
        }
        return res;
    }
}