leetcode(18)四数之和

四数之和

解题思路：排序+Hash+双指针

class Solution {
        public List<List<Integer>> fourSum(int[] nums, int target) {
        int len =  nums.length;
        Arrays.sort(nums);
        Map<Integer,Integer> map = new HashMap<>();
        for(int i=0;i<len;i++){
            map.put(nums[i],i);
        }
        int l = 0;
        int r = 0;
        int temp = 0;
        int target2 = 0;
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        List<Integer> list = null;
        int count = 1;
        int root = 1;
        for(int i=0;i<len-3;i++){
            if(i+1<len-3+root-1&&nums[i+1]==nums[i]){
                root++;
                if(root==3){//这里求的是前三个或者前四个数相等的情况
                    target2 = target - 3*nums[i];
                    if(map.get(target2)!=null&&map.get(target2)>i+1){
                        list = new ArrayList<Integer>();
                        list.add(nums[i]);
                        list.add(nums[i]);
                        list.add(nums[i]);
                        list.add(target2);
                        result.add(list);
                    }
                }
                continue;
            }
            if(root>=2){//这里求的是前两个数相等的情况
                l=i+1;
                r=len-1;
                target2 = target - 2*nums[i];
                if(l<r&&nums[r-1]+nums[r]<target2&&nums[l]+nums[l+1]>target2){
                    
                }else{
                    while(l<r){
                        temp = nums[l]+nums[r];
                        if(temp>target2){
                            while(l<r&&nums[r-1]==nums[r]){
                                r--;
                            }
                            r--;
                        }else if(temp<target2){
                            while(l<r&&nums[l+1]==nums[l]){
                                l++;
                            }
                            l++;
                        }else{
                            list = new ArrayList<Integer>();
                            list.add(nums[i]);
                            list.add(nums[i]);
                            list.add(nums[r]);
                            list.add(nums[l]);
                            result.add(list);
                            while(l<r&&nums[r-1]==nums[r]){
                                r--;
                            }
                            r--;
                            while(l<r&&nums[l+1]==nums[l]){
                                l++;
                            }
                            l++;
                        }
                    }
                }
            }
            root=1;
            count =1;
            for(int j=i+1;j<len-2;j++){
                if(j+1<len-2+count-1&&nums[j+1]==nums[j]){
                    count++;
                    if(count==2){//后三个数相等或者中间两个数相等的情况
                        target2 = target - nums[i] - 2*nums[j];
                        if(map.get(target2)!=null&&map.get(target2)>j+1){
                            list = new ArrayList<Integer>();
                            list.add(nums[i]);
                            list.add(nums[j]);
                            list.add(nums[j]);
                            list.add(target2);
                            result.add(list);
                        }
                    }
                    continue;
                }
                count=1;
                l = j+1;
                r = len-1;
                target2 = target - nums[i] - nums[j];
                if(l<r&&nums[r-1]+nums[r]<target2&&nums[l]+nums[l+1]>target2){
                    
                }else{
                    while(l<r){//四个数都不相等或者后两个数相等的情况
                        temp = nums[l]+nums[r];
                        if(temp>target2){
                            while(l<r&&nums[r-1]==nums[r]){
                                r--;
                            }
                            r--;
                        }else if(temp<target2){
                            while(l<r&&nums[l+1]==nums[l]){
                                l++;
                            }
                            l++;
                        }else{
                            list = new ArrayList<Integer>();
                            list.add(nums[i]);
                            list.add(nums[j]);
                            list.add(nums[r]);
                            list.add(nums[l]);
                            result.add(list);
                            while(l<r&&nums[r-1]==nums[r]){
                                r--;
                            }
                            r--;
                            while(l<r&&nums[l+1]==nums[l]){
                                l++;
                            }
                            l++;
                        }
                    }
                }
            }
            
        }
        return result;
    }
}