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)

方法

先将数组排序。从头開始选择第一个元素。然后在选择剩下的两个元素。

    public List<List<Integer>> threeSum(int[] num) {
        List<List<Integer>> list = new ArrayList<List<Integer>>();
         Arrays.sort(num);
        int len = num.length;
        for (int i = 0; i < len - 2; i++) {
            if (i == 0 || num[i] != num[i - 1]) {
                int target = 0 - num[i];
                int left = i + 1;
                int right = len - 1;
                while (left < right) {
                    if (left < right) {
                        int temp = num[left] + num[right];
                        if (temp == target) {
                        	if (left == i + 1 || num[left] != num [left - 1]) {
                                List<Integer> subList = new ArrayList<Integer>();
                                subList.add(num[i]);
                                subList.add(num[left]);
                                subList.add(num[right]);
                                list.add(subList);
                        	}
                            left++;
                            right--;
                        } else if (temp > target) {
                            right--;
                        } else {
                            left++;
                        }
                    }
    
                }                
            }

        }
        return list;
    }