leetcode题

1、一个数组nums，输入target，计算nums中哪两个索引下的值的和等于target,并输出两个的索引值

public int[] twoSum(int[] nums, int target) {    Map<Integer, Integer> map = new HashMap<>();    for (int i = 0; i < nums.length; i++) {        int complement = target - nums[i];        if (map.containsKey(complement)) {            return new int[] { map.get(complement), i };        }        map.put(nums[i], i);    }    throw new IllegalArgumentException("No two sum solution");} 

2、计算两个数的汉明距离，汉明距离为两个数的二进制相比较，有几位不同

public class Solution {    public static void main(String[] args) {        System.out.println(hammingDistance(4,1));    }    public static int hammingDistance(int x, int y) {        int z = x^y;        int num = 0;        while(z!=0){            if(z%2==1){                num++;            }            z/=2;        }        return num;    }}

3、计算补码对应的32位二进制

    public static int findComplement(int num) {        return ~num & ((Integer.highestOneBit(num) << 1) - 1);    }

4、Given an array of 2n integers, your task is to group these integers into n pairs of integer, say (a1, b1), (a2, b2), ..., (an, bn) which makes sum of min(ai, bi) for all i from 1 to n as large as possible.

给定一个数组，长度为2n，把他们分成n组，使min(ai,bi)和相加尽可能的大

Example 1:

Input: [1,4,3,2]Output: 4Explanation: n is 2, and the maximum sum of pairs is 4.

Note:

1. n is a positive integer, which is in the range of [1, 10000].
2. All the integers in the array will be in the range of [-10000, 10000]

思路：先进行排序，在取相邻的两个数取最小值进行想加

public class Solution {    public static void main(String[] args){        int[] nums = {1,4,3,2,5,6};        int i = arrayPairSum(nums);        System.out.print(i);    }    public static int arrayPairSum(int[] nums) {        Arrays.sort(nums);        int sum = 0;        for(int i=0;i<nums.length;i+=2){            if(nums[i]<nums[i+1]){                sum = sum + nums[i];            }else{                sum = sum + nums[i+1];            }        }        return sum;    }}

Given a string, you need to reverse the order of characters in each word within a sentence while still preserving whitespace and initial word order.

Example 1:

Input: "Let's take LeetCode contest"Output: "s'teL ekat edoCteeL tsetnoc"

Note: In the string, each word is separated by single space and there will not be any extra space in the string.

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public class Solution {    public static void main(String[] args) {        System.out.print(reverseWords("Let's take LeetCode contest"));    }    public static String reverseWords(String s) {        StringBuilder sb = new StringBuilder();        String[] str = s.split(" ");        for(int i=0;i<str.length;i++){            StringBuilder strs = new StringBuilder();            strs.append(str[i]);            strs.reverse();            if(i==str.length-1){                sb.append(strs);            }else{                sb.append(strs+" ");            }        }        return sb.toString();    }}