Sum—LeetCode-15 3Sum

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问题描述：

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]]

解题思想：固定一个值，就可以转化为两个数和等于指定值，要学会抽象思维，时间复杂度O(n*n)，见代码：

public List<List<Integer>> threeSum(int[] nums) {        List<List<Integer>> resList = new ArrayList<List<Integer>>();        Arrays.sort(nums);        int pre = Integer.MAX_VALUE;        for(int i = 0;i < nums.length-2; ++i){            int target = nums[i] * (-1);            if(pre != nums[i]){                List<List<Integer>> tempList = twoSum(nums, i+1, target);                if(null != tempList){                    for(List<Integer> list: tempList){                        List<Integer> temp = new ArrayList<Integer>();                        temp.add(nums[i]);                        temp.add(list.get(0));                        temp.add(list.get(1));                        resList.add(temp);                    }                }            }            pre = nums[i];        }        return resList;    }    public List<List<Integer>> twoSum(int[] nums, int beginIndex, int target){        List<List<Integer>> resList = new ArrayList<List<Integer>>();        int end = nums.length - 1;        int preBeginElem = Integer.MAX_VALUE;        int preEndElem = Integer.MAX_VALUE;        while(beginIndex < end) {            if(nums[beginIndex] + nums[end] > target){                --end;            } else if(nums[beginIndex] + nums[end] < target){                ++beginIndex;            } else {                if(nums[beginIndex] != preBeginElem && nums[end] != preEndElem){                    List<Integer> tempList = new ArrayList<Integer>();                    tempList.add(nums[beginIndex]);                    tempList.add(nums[end]);                    resList.add(tempList);                }                preBeginElem = nums[beginIndex];                preEndElem = nums[end];                ++beginIndex;                --end;            }        }        return resList;    }

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