LeetCode | 3Sum

题目

Given 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:

• Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
• 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)

分析

注意去重！

代码

import java.util.ArrayList;
import java.util.Arrays;

public class ThreeSum {
	public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
		ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
		Arrays.sort(num);
		for (int i = 0; i < num.length - 2 && num[i] <= 0; ++i) {
			if (i > 0 && num[i] == num[i - 1]) {
				continue;
			}
			int target = -num[i];
			int j = i + 1;
			int k = num.length - 1;
			while (j < k) {
				if (j > i + 1 && num[j] == num[j - 1]) {
					++j;
					continue;
				}
				if (k < num.length - 1 && num[k] == num[k + 1]) {
					--k;
					continue;
				}
				int sum = num[j] + num[k];
				if (sum < target) {
					++j;
				} else if (sum > target) {
					--k;
				} else {
					ArrayList<Integer> list = new ArrayList<Integer>();
					list.add(num[i]);
					list.add(num[j]);
					list.add(num[k]);
					results.add(list);
					++j;
				}
			}
		}
		return results;
	}
}