15. 3Sum

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: 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]

]

public class Solution {

public List<List<Integer>> threeSum(int[] num) {

List<List<Integer>> result = new ArrayList<List<Integer>>();

if(num==null || num.length<3)

return result;

Arrays.sort(num);

for(int i=0; i<num.length-2; i++) {

if(i>0 && num[i]==num[i-1]) continue;

int left = i+1;

int right = num.length - 1;

while(left<right) {

int sum = num[left] + num[right];

if(sum == -num[i]) {

List<Integer> r = new ArrayList<Integer>();

r.add(num[i]);

r.add(num[left]);

r.add(num[right]);

result.add(r);

left++;

right--;

while(left<right && num[left]==num[left-1]) left++;

while(right>left && num[right]==num[right+1]) right--;

} else if(sum < -num[i]) {

left++;

} else {

right--;

}

}

}

return result;

}

}