15. 3Sum

1 題目

iven an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

2 代碼

public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
		Arrays.sort(nums);
		List<List<Integer>> lists = new LinkedList<>();

		int p, q, sum;
		for (int i = 0; i < nums.length - 2; i++) {
			if (i > 0 && nums[i] == nums[i - 1]) {
				continue;
			}

			p = i + 1;
			q = nums.length - 1;

			while (p < q) {
				sum = nums[i] + nums[p] + nums[q];

				if (sum > 0) {
					q = uniqueIndex2(nums, q);
				} else if (sum < 0) {
					p = uniqueIndex1(nums, p);
				} else {
					List<Integer> list = new LinkedList<>();
					list.add(nums[i]);
					list.add(nums[p]);
					list.add(nums[q]);
					lists.add(list);
					q = uniqueIndex2(nums, q);
					p = uniqueIndex1(nums, p);
				}
			}
		}

		return lists;
	}

	public int uniqueIndex1(int[] nums, int index) {
		while ((index < nums.length - 1) && nums[index] == nums[index + 1]) {
			index++;
		}
		return index + 1;
	}

	public int uniqueIndex2(int[] nums, int index) {
		while ((index > 0) && (nums[index] == nums[index - 1])) {
			index--;
		}
		return index - 1;
	}
}