18. 四数之和 - 中等

跟三数之和差不多，不过

双指针法

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> list = new ArrayList<>();
        if(nums.length < 4) return list;
        Arrays.sort(nums);
        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++){
                // j > i + 1,别写成j > 1
                if(j > i + 1 && nums[j] == nums[j-1]) continue;
                int left = j+1;
                int right = nums.length - 1;
                int remain = target - nums[i] - nums[j];
                while(left < right){
                    int temp = nums[left] + nums[right];
                    if(temp == remain){
                        list.add(Arrays.asList(nums[i], nums[j], nums[left] , nums[right]));
                        right--;
                        left++;
                        while(left < right && nums[right] == nums[right+1]) right--;
                        while(left < right && nums[left] == nums[left - 1]) left++;
                    } 
                    else if(temp > remain) right--;
                    else left++;
                }
            }
        }
        return list;
    }
}

多加两行

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> list = new ArrayList<>();
        if(nums.length < 4) return list;
        Arrays.sort(nums);
        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++){
                // j > i + 1,别写成j > 1
                if(j > i + 1 && nums[j] == nums[j-1]) continue;
                int left = j+1;
                int right = nums.length - 1;
                int remain = target - nums[i] - nums[j];
                //以下两行大大减少没必要的遍历
                if(nums[left] + nums[left+1] > remain) break; 
                if(nums[right] + nums[right-1] < remain) continue;//这里不是break
                while(left < right){
                    int temp = nums[left] + nums[right];
                    if(temp == remain){
                        list.add(Arrays.asList(nums[i], nums[j], nums[left] , nums[right]));
                        right--;
                        left++;
                        while(left < right && nums[right] == nums[right+1]) right--;
                        while(left < right && nums[left] == nums[left - 1]) left++;
                    } 
                    else if(temp > remain) right--;
                    else left++;
                }
            }
        }
        return list;
    }
}