摘要:解题思路题目要求两个数和等于,返回其题目说明不会有重复情况,所以我们一旦发现符合情况的,就可以直接结束循环并返回。特殊情况就是正好等于,那肯定是最接近的情况,直接返回即可。
Two Sum
Given an array of integers, return indices of the two numbers such that they add up to a specific target.
You may assume that each input would have exactly one solution.
Example: Given nums = [2, 7, 11, 15], target = 9, Because nums[0] + nums[1] = 2 + 7 = 9, return [0, 1].
1.解题思路
题目要求两个数和等于target,返回其index.题目说明不会有重复情况,所以我们一旦发现符合情况的,就可以直接结束循环并返回。
利用HashMap
2.代码
public class Solution { public int[] twoSum(int[] nums, int target) { int[] res=new int[2]; if(nums.length==0) return res; HashMaphm=new HashMap (); for(int i=0;i 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: 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] ]1.解题思路
本题要求三个数的和为0,其实就是固定一个数n之后,找两个数和为0-n,所以就转化为求两个数的和,那我们很容易想到使用二分法。
注意点:本题需要排除重复值,
1)固定的数下标为i,则要从i+1开始寻找后两个数;
2)如果数组中包含重复的数,则为了保证结果不出现重复,我们每次遇到重复的数需要跳过。2.代码
public class Solution { List> res =new ArrayList
>(); public List
> threeSum(int[] nums) { Arrays.sort(nums); for(int i=0;i
0&&nums[i]==nums[i-1]) continue; //避免重复 twoSum(0-nums[i],nums,i+1,nums.length-1); } return res; } private void twoSum(int target,int[] nums, int start,int end){ int i=start,j=end; while(i subres=new ArrayList (); int sum=nums[i]+nums[j]; if(sum==target){ subres.add(0-target); subres.add(nums[i]); subres.add(nums[j]); res.add(subres); do { i++; }while(i < end && nums[i] == nums[i-1]); do { j--; } while(j >= 0 && nums[j] == nums[j+1]); } else if(sum 4Sum
Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.Note: The solution set must not contain duplicate quadruplets.
For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0. A solution set is: [ [-1, 0, 0, 1], [-2, -1, 1, 2], [-2, 0, 0, 2] ]1.解题思路
4Sum,就是在3Sum基础上再嵌套一层,注意点也是要避免重复。
2.代码
public class Solution { ArrayList> res = new ArrayList
>(); public List
> fourSum(int[] nums, int target) { Arrays.sort(nums); for(int i=0;i
0&&nums[i]==nums[i-1]) continue; res.addAll(threesum(target-nums[i],nums,i+1)); } return res; } private List > threesum(int target,int[] nums,int start){ ArrayList
> res = new ArrayList
>(); int first=start-1; for(int i=start;i
start&&nums[i]==nums[i-1]) continue; res.addAll(twosum(target-nums[i],nums,i+1,first)); } return res; } private List > twosum(int target,int[] nums,int start,int first){ List
> res =new ArrayList
>(); int second=start-1; int i=start; int j=nums.length-1; while(i
subres =new ArrayList (); int sum=nums[i]+nums[j]; if(sum==target){ subres.add(nums[first]); subres.add(nums[second]); subres.add(nums[i]); subres.add(nums[j]); res.add(subres); i++; while(i =0&&nums[j]==nums[j+1]){ j--; } } else if(sum>target) j--; else i++; } return res; } } 3Sum Closest
Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.For example, given array S = {-1 2 1 -4}, and target = 1. The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).1.解题思路
与3Sum相似,只不过本次是要寻找最接近target的3个数之和,我们需要维护minDiff和closetSum两个变量。
但是本题题目说明不许考虑重复。 特殊情况就是sum正好等于target,那肯定是最接近的情况,直接返回即可。2.代码
public class Solution { public int threeSumClosest(int[] nums, int target) { if(nums.length==0) return 0; int minDiff=Integer.MAX_VALUE; int closet=0; Arrays.sort(nums); for(int i=0;itarget){ right--; } else return sum; } } return closet; } } 4Sum II
Given four lists A, B, C, D of integer values, compute how many tuples (i, j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.
To make problem a bit easier, all A, B, C, D have same length of N where 0 ≤ N ≤ 500. All integers are in the range of -228 to 228 - 1 and the result is guaranteed to be at most 231 - 1.
Example: Input: A = [ 1, 2] B = [-2,-1] C = [-1, 2] D = [ 0, 2] Output: 2 Explanation: The two tuples are: 1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0 2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 01.解题思路
本题从1个数组扩展到了4个数组,求的是有多少组4数之和等于target的,我们把问题分为两个数一组,假设当前两个数A[i]+B[j],那我们其实就是要在C和D中求是否存在和为0-A[i]-B[j],如果存在则返回存在的个数。想到用HashMap,存储
. 2.代码
public class Solution { public int fourSumCount(int[] A, int[] B, int[] C, int[] D) { if(A.length==0||B.length==0||C.length==0||D.length==0) return 0; Mapmap=new HashMap (); int count=0; for(int i=0;i
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