LeetCode 18. 4Sum [Medium]

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题目来源：https://leetcode.com/problems/4sum/

题目难度：Medium

解答1[Java]

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class Solution {
    public List<List<Integer>> fourSum(int[] num, int target) {
        ArrayList<List<Integer>> ans = new ArrayList<>();
        if (num.length < 4)
            return ans;
        Arrays.sort(num);
        for (int i = 0; i < num.length - 3; i++) {
            if (num[i] + num[i + 1] + num[i + 2] + num[i + 3] > target)
                break; // first candidate too large, search finished
            if (num[i] + num[num.length - 1] + num[num.length - 2] + num[num.length - 3] < target)
                continue; // first candidate too small
            if (i > 0 && num[i] == num[i - 1])
                continue; // prevents duplicate result in ans list
                          // already sorted, only need to check the adjacent element

            for (int j = i + 1; j < num.length - 2; j++) {
                if (num[i] + num[j] + num[j + 1] + num[j + 2] > target)
                    break; // second candidate too large
                if (num[i] + num[j] + num[num.length - 1] + num[num.length - 2] < target)
                    continue; // second candidate too small
                if (j > i + 1 && num[j] == num[j - 1])
                    continue; // prevents duplicate results in ans list
                int low = j + 1, high = num.length - 1;
                while (low < high) {
                    int sum = num[i] + num[j] + num[low] + num[high];
                    if (sum == target) {
                        ans.add(Arrays.asList(num[i], num[j], num[low], num[high]));
                        while (low < high && num[low] == num[low + 1])
                            low++; // skipping over duplicate on low
                        while (low < high && num[high] == num[high - 1])
                            high--; // skipping over duplicate on high
                        low++;
                        high--;
                    }
                    // move window
                    else if (sum < target)
                        low++;
                    else
                        high--;
                }
            }
        }
        return ans;
    }
}