18.4-sum.java

/*
 * @lc app=leetcode id=18 lang=java
 *
 * [18] 4Sum
 */

// @lc code=start
class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> res = new ArrayList<>();
        Arrays.sort(nums);
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] > 0 && nums[i] > target) {
                return res;
            }
            if (i > 0 && nums[i - 1] == nums[i]) {
                continue;
            }
            for (int j = i + 1; j < nums.length; j++) {
                if (j > i + 1 && nums[j - 1] == nums[j]) {
                    continue;
                }
                int left = j + 1;
                int right = nums.length - 1;
                while (right > left) {
                    long sum = (long) nums[i] + nums[j] + nums[left] + nums[right];
                    if (sum > target) {
                        right--;
                    } else if (sum < target) {
                        left++;
                    } else {
                        res.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));
                        while (right > left && nums[right] == nums[right - 1]) right--;
                        while (right > left && nums[left] == nums[left + 1]) left++;
                        left++;
                        right--;
                    }
                }
            }
        }
        return res;
    }
}
// @lc code=end