package amazon;

// 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]

// ]

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

/**

* Created by anduo on 17-3-13.

*/

public class ThreeSum {

// 把3sum的问题，降级到2sum问题

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

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

Arrays.sort(nums);

for (int i = 0; i < nums.length; i++) {

if (i > 0 && nums[i] == nums[i - 1]) {// 这道题用不到这个条件，因为题目说了不考虑重复元素

continue;

}

int left = i + 1;

int right = nums.length - 1;

int target = -nums[i];

twoSum(nums, left, right, target, result);

}

return result;

}

public void twoSum(int[] nums,

int left,

int right,

int target,

List<List<Integer>> results) {

// 充分利用有序这样一个特点

while (left < right) {

if (nums[left] + nums[right] == target) {

ArrayList<Integer> triple = new ArrayList<>();

triple.add(-target);

triple.add(nums[left]);

triple.add(nums[right]);

results.add(triple);

left++;

right--;

// skip duplicate pairs with the same left

while (left < right && nums[left] == nums[left - 1]) {

left++;

}

// skip duplicate pairs with the same right

while (left < right && nums[right] == nums[right + 1]) {

right--;

}

} else if (nums[left] + nums[right] < target) {

left++;

} else {

right--;

}

}

}

}