LeetCode-3SUM（数组中三数之和为0）

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)

第一想法肯定是三层循环遍历的，时间复杂度O(n^3)，但是做过了了 Two Sum ，感觉瞬间可解，随用hashtable的方法：

public List<List<Integer>> threeSum1(int[] num) {
        List<List<Integer>> lists = new ArrayList<>();
        HashMap<Integer, Integer> hash = new HashMap<>();
        for (int i = 0; i < num.length; i++) {
        	hash.put(num[i], i);
        }
        for (int i = 0; i < num.length; i++) {
        	for (int j = i+1; j < num.length; j++) {
        		if (hash.get(0-num[i]-num[j]) != null && num[i] <= num[j] && num[j] <= 0-num[i]-num[j]) {
        			List<Integer> list = new ArrayList<>();
        			list.add(num[i]);
        			list.add(num[j]);
        			list.add(0-num[i]-num[j]);
        			lists.add(list);
        		}
        	}
        }
        return lists;
    }

先上代码：

public List<List<Integer>> threeSum(int[] num) {
    	List<List<Integer>> lists = new ArrayList<>();
    	Arrays.sort(num);
    	for (int i = 0; i < num.length - 2; i++) {
    		if (i == 0 || (i > 0 && num[i] != num[i-1])) {
    			int low = i+1, high = num.length-1, sum = 0-num[i];
    			while (low < high) {
    				if (num[low]+num[high] == sum) {
    					lists.add(Arrays.asList(num[i], num[low], num[high]));
    					while (low < high && num[low] == num[low+1]) {
    						low ++;
    					}
    					while (low <high && num[high] == num[high-1]) {
    						high --;
    					}
    					low++;
    					high--;
    				} else if (num[low] + num[high] < sum) {
    					low++;
    				} else {
    					high--;
    				}
    			}
    		}
    	}
    	return lists;
    }

先控制一个变量a，线性遍历。此时只需找出b,c满足b+c=-a即可。由于是排序好的，剩下的可以用二分查找的思想找出b.c。于是求解（注意要排除重复解！！！！）

原文：http://blog.csdn.net/my_jobs/article/details/43603901