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:

• 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)

思路： We need to find every number's negative number which is two numbers sum of the rest numbers. Then set up a start = i + 1, end = num.length - 1; If sum less than the negate, start ++ , else end--; if find a equal , add the solution. then start++ and end--.

易错点： 1. 为了不重复， 一定要i == 0 || num[i] > num[i - 1].  2 为了不重复， 再找到之后start++, end--, 然后 用while(start < end && num[end] == num[end + 1]) 和 while(start < end && num[start] == num[start - 1]) 来确保不是重复的解。

public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
	    if (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]){
	            int negate = -num[i];
	            int start = i + 1;
	            int end = num.length - 1;
	            while(start < end){
	                int sum = num[start] + num[end];
	                if(sum == negate){
	                    ArrayList<Integer> sol = new ArrayList<Integer>();
	                    sol.add(num[i]);
	                    sol.add(num[start]);
	                    sol.add(num[end]);
	                    result.add(sol);
	                    start++;
	                    end--;
	                    while(start < end && num[end] == num[end + 1])
	                        end--;
	                    while(start < end && num[start] == num[start - 1])
	                        start++;
	                }
	                if(sum < negate)
	                    start++;
	                if(sum > negate)
	                    end--;
	            }
	        }
	    }
	    return result;
    }
}