[LeetCode]3Sum,解題報告

題目

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.

思路

開始想用深搜解決，後來發現用深搜很難保證結果沒有重復的結合，因此采用排序+雙指針固定解決思路，同時增加去除重復的判斷機制

AC代碼

public class Solution {
    public List> threeSum(int[] num) {
        List> res = new ArrayList>();

        if (num.length < 3) {
            return res;
        }

        Arrays.sort(num);

        for (int i = 0; i < num.length - 2; i++) {
            // avoid duplicate
            if (i == 0 || num[i] > num[i - 1]) {
                int start = i + 1;
                int end = num.length - 1;

                while (start < end) {
                    int stand = num[i] + num[start] + num[end];
                    if (stand == 0) {
                        ArrayList tmp = new ArrayList();
                        tmp.add(num[i]);
                        tmp.add(num[start]);
                        tmp.add(num[end]);
                        res.add(tmp);
                        start++;
                        end--;
                        //avoid duplicate
                        while (start < end && num[start] == num[start - 1]) {
                            start ++;
                        }
                        while (start < end && num[end] == num[end + 1]) {
                            end --;
                        }
                    } else if (stand > 0) {
                        end--;
                    } else {
                        start++;
                    }
                }
            }
        }

        return res;
    }
}

後記

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