两数之和

package com.leetcode.part1;

import java.util.HashMap;
import java.util.Map;

/**
 * @author ：shix
 * @date ：Created in 2020/1/26 12:56
 * @description：两数之和
 * @modified By：
 * @version: 1.0.0$
 */
public class SumOfTwoNumbers {

    /**
     * 解方一
     * 简单粗暴，时间复杂度为O（n^2）
     *
     * @param nums
     * @param target
     * @return
     */
    public static int[] getIndexOfTwoNumbers(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("There is not two numbers");
    }

    /**
     * 解法二：
     * 哈希表：保持数组中的每个元素与其索引相互对应的最好方法是什么？哈希表。
     * 前提条件：数组无重复元素
     *
     * @param nums
     * @param target
     * @return
     */
    public static int[] getIndexOf2Numbers(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            map.put(nums[i], i);
        }
        for (int i = 0; i < nums.length; i++) {
            int temp = target - nums[i];
            if (map.containsKey(temp) && map.get(temp) != i) {
                return new int[]{map.get(temp), i};
            }
        }
        throw new IllegalArgumentException("There is not two numbers");
    }

    /**
     * 解法三
     * 一次hash循环
     *
     * @param nums
     * @param target
     * @return
     */
    public static int[] getIndexOf2NumsOneLoop(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int temp = target - nums[i];
            if (map.containsKey(temp)) {
                return new int[]{map.get(temp), i};
            }
            map.put(nums[i], i);
        }
        throw new IllegalArgumentException("There is not two numbers");
    }

    public static void main(String[] args) {
        int[] nums = {2, 7, 11, 15};
        int target = 9;
        // 解法一
//        int[] results = getIndexOfTwoNumbers(nums, target);
        // 解法二
//        int[] results = getIndexOf2Numbers(nums, target);
        // 解法三
        int[] results = getIndexOf2NumsOneLoop(nums, target);
        System.out.println("和为:" + target + "的两数下标为【" + results[0] + "," + results[1] + "】");
    }


}