15. 3Sum

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 first = 0;first<num.length-2;++first)
        {
            if(first > 0 && num[first]==num[first-1])
                continue;
            if(num[first]>0)
                break;
            
            int second = first+1;
            int third = num.length -1;
            
            while(second<third)
            {
                int sum = num[first]+num[second]+num[third]; 
                if(sum == 0)
                {
                    List<Integer> l = new ArrayList<Integer>();
                    l.add(num[first]);
                    l.add(num[second]);
                    l.add(num[third]);
                    result.add(l);
                    while(++second<third && num[second]==num[second-1]);
                    while(second<--third && num[third]==num[third+1]);
                }
                else if(sum>0)
                    while(second<--third && num[third]==num[third+1]);
                else 
                    while(++second<third && num[second]==num[second-1]);
            }
            
        }
        
        return result;
    }
}