文章--LeetCode算法--TwoSum

目录

• TwoSum
  • 问题描述
  • 示例
  • 解决思路
  • 实现代码

TwoSum

问题描述

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, and you may not use the same element twice.

示例

Example:
Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

解决思路

思路：查找时，建立索引（Hash查找）或进行排序（二分查找）。本题缓存可在找的过程中建立索引，故一个循环可以求出解（总是使用未
使用元素查找使用元素，可以保证每一对都被检索到）。Indexing/ordering is the first step to search questions.

实现代码

package com.leetcode.play;

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

/**
 * 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, and you may not use the same element twice.
 * Example:
 * Given nums = [2, 7, 11, 15], target = 9,
 * Because nums[0] + nums[1] = 2 + 7 = 9,
 * return [0, 1].
 * @since  1.8
 */

public class TwoSum {

    public static void main(String[] args) {
        int[] nums = {2, 7, 11, 15};
        int target = 9 ;
        int[] ints = twoSum(nums,target);
        for (int i:ints) {
            System.out.println(i);
        }


    }

    public static int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> m = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; ++i) {
            int num = target - nums[i];
            if (m.containsKey(num))
                return new int[] {m.get(num), i};
            m.put(nums[i], i);
        }
        throw new RuntimeException("no answer!");
    }





}