（一天一练）算法学习

题1：给定一个数组和一个目标和，从数组中找两个数字相加等于目标和，输出这两个数字的下标。
Given nums = [2,7,11,15],target =9,

Because nums[0] +nums[1] = 2+7 = 9,

return [0,1]

 

package test;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class TestController01 {
    /**
     * 
     */
    public static void main(String[] args) {
        int[] nums = {2,7,11,15,1,8};
        int target = 9;
//        int[] twoSum = twoSum1(nums,target);
//        int[] twoSum = twoSum2(nums,target);
        List<Integer> twoSum = twoSum3(nums,target);
        for (int i : twoSum) {
            System.out.println(i);
        }
    }
    
    /**
     * 方法1：两层循环
     * @param nums
     * @param target
     * @return
     */
    public static int[] twoSum1(int[]nums,int target) {
        int[] ans = new int[2];
         for (int i = 0; i < nums.length; i++) {
            for (int j = i+1; j < nums.length; j++) {
                if(nums[i]+nums[j] == target) {
                    ans[0] = i;
                    ans[1] = j;
                }
            }
        }
        return ans;    
    }
    
    /**
     * 方法2：不用遍历就能找出
     * @param nums
     * @param target
     * @return
     */
    public static int[] twoSum2(int[] nums, int target) {
        int[] ans = new int[2];
        Map<Integer,Integer> map = new HashMap<Integer,Integer>();
        for (int i = 0; i < ans.length; i++) {
            map.put(nums[i],i);
        }
        for (int i = 0; i < ans.length; i++) {
            int two = target - nums[i];
//            if(map.containsKey(two) && target != 2*two) {
//                ans[0] = map.get(two);
//                ans[1] = i;
//                return ans;
//            }
            if(map.containsKey(two)&&map.get(two)!=i){
                return new int[]{i,map.get(two)};
            }
        }
        throw new IllegalArgumentException("No two sum solution");
    }
    
    
    /**
     * 方法3：对方法2分析，方法2中存在两个同样的for循环，可以对此进行合并，只是不需要判断是否是当前元素了
     * @param nums
     * @param target
     * @return
     */
    public static List<Integer> twoSum3(int[] nums, int target) {
        List<Integer> twoList = new ArrayList<>();
        Map<Integer,Integer> map = new HashMap<Integer,Integer>();
        for (int i = 0; i < nums.length; i++) {
            int two = target - nums[i];
            if(map.containsKey(two)) {
                twoList.add(map.get(two));
                twoList.add(i);
            }
            map.put(nums[i], i);
        }
        return twoList;
    }
}