Two Sum（给定一个值，找出一个数组中“和”为该值的数值组合）

@sometwo.fun

于 2018-04-08 20:16:50 发布

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文章标签： code 算法珠玑 array

本文链接：https://blog.youkuaiyun.com/Json0108/article/details/79857146

本文介绍了一种解决两数之和问题的有效算法。该算法通过使用哈希表来存储数组元素及其对应的索引，实现了快速查找目标和所需配对元素的功能。输入为整数数组及目标值，输出为配对元素的索引（非零基）。文章提供了完整的Java实现代码及运行示例。

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import java.util.HashMap;

/**
 * Two Sum
 *
 * Given an array of integers, find two numbers such 
 * that they add up to a specific target number.
 * The function twoSum should return indices of the
 * two numbers such that they add up to the target,
 * where index1 must be less than index2.
 * Please note that your returned answers (both index1 and index2) are not zero-based.
 *
 * You may assume that each input would have exactly one solution.
 *
 * Input:  numbers={2, 7, 11, 15}, target=9
 * Output:  index1=1, index2=2
 */
public class TwoSum {
    public static int[] twoSum(int[] numbers, int target) {
        if(numbers == null) return null;
        final HashMap<Integer, Integer> set = new HashMap<Integer, Integer>();
        for(int i=0; i<numbers.length; i++) {
            set.put(numbers[i], i);
        }
        for(int i=0; i<numbers.length; i++) {
            Integer value = set.get(target - numbers[i]);
            if(value==null || value==i) {
                continue;
            }
            if(value > i) {
                return new int[]{++i, ++value};
            } else if(value < i) {
                return new int[]{++value, ++i};
            }
        }
        return null;
    }

    public static void main(String[] args) {
        int[] arr = {2, 7, 11, 15};
        int[] result = twoSum(arr, 9);
        for(int i : result) {
            System.out.print(i + " ");
        }
    }
}