leetcode 1. Two Sum

原创于 2017-05-11 14:18:18 发布 · 214 阅读

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本文介绍了一种解决两数之和问题的有效算法。给定一个整数数组及目标值，该算法能快速找到两个数的下标，这两个数相加等于目标值。通过使用哈希表，可以实现平均时间复杂度为O(n)的解决方案。

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].

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer,Integer> map=new HashMap<Integer,Integer>();
        int[] a=new int[2];
         for(int i=0;i<nums.length;i++){
             if(map.get(target-nums[i])==null){
                 map.put(nums[i],i);
             }else{
                 a[0]=map.get(target-nums[i]);
                 a[1]=i;
             }
         }
         return a;
    }
}