暑假牛客网刷题day1:Java数学之美（一）

import java.util.*;
public class Solution {
    public int[] twoSum(int[] nums, int target) {
        for(int i = 0;i < nums.length;i++) {
            for(int j = i + 1;j < nums.length;j++) {
                if(nums[i] + nums[j] == target)
                    return new int[]{i,j};
            }
        }

        throw new IllegalArgumentException("No two sum solution");
    }
}

import java.util.*;
public class Solution {
    /**
     * 
     * @param numbers int整型一维数组 
     * @param target int整型 
     * @return int整型一维数组
     */
    public int[] twoSum (int[] numbers, int target) {
        // write code here
        HashMap<Integer, Integer> map = new HashMap<>();
        //遍历数组
        for (int i = 0; i < numbers.length; i++) {
            //将不包含target - numbers[i]，装入map中，包含的话直接返回下标
            if(map.containsKey(target - numbers[i])) 
                return new int[]{map.get(target - numbers[i])+1, i+1};
            else 
                map.put(numbers[i], i);
        }
        throw new IllegalArgumentException("No solution");
    }
}

import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        ArrayList<ArrayList<Integer> > res = new ArrayList<ArrayList<Integer>>();
        int n = num.length;
        //不够三元组 fast-template
        if(n < 3)
            return res;
        //排序
        Arrays.sort(num);
        for(int i = 0; i < n - 2; i++){
            if(i != 0 && num[i] == num[i - 1])
                continue;
            //后续的收尾双指针
            int left = i + 1;
            int right = n - 1;
             //设置当前数的负值为目标
            int target = -num[i];
            while(left < right){
                //双指针指向的二值相加为目标，则可以与num[i]组成0
                if(num[left] + num[right] == target){
                    ArrayList<Integer> temp = new ArrayList<Integer>();
                    temp.add(num[i]);
                    temp.add(num[left]);
                    temp.add(num[right]);
                    res.add(temp);
                    while(left + 1 < right && num[left] == num[left + 1])
                        //去重
                        left++;
                    while(right - 1 > left && num[right] == num[right - 1])
                        //去重
                        right--;
                    //双指针向中间收缩
                    left++;
                    right--;
                }
                //双指针指向的二值相加大于目标，右指针向左
                else if(num[left] + num[right] > target)
                    right--;
                //双指针指向的二值相加小于目标，左指针向右
                else left++;
            }
        }
        return res;}
}