[LeetCode]3Sum,解题报告

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.

思路

开始想用深搜解决，后来发现用深搜很难保证结果没有重复的结合，因此采用排序+双指针固定解决思路，同时增加去除重复的判断机制

AC代码

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

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<Integer> tmp = new ArrayList<Integer>();
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;
}
}

后记

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