LeetCode Everyday: 1. Two Sum

jimhust

于 2016-05-11 20:31:00 发布

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分类专栏： leetcode 文章标签： leetcode

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本文介绍了一种高效解决两数之和问题的算法。通过使用哈希表记录已访问过的数字及其索引，该算法能在O(n)的时间复杂度内找到数组中两个数相加等于特定目标值的元素索引。

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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.

Example:

Given nums = [2, 7, 11, 15], target = 9,

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

UPDATE (2016/2/13):

The return format had been changed to zero-based indices. Please read the above updated description carefully.

import java.util.*;
public class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashMap<Integer,Integer> hm = new HashMap<Integer,Integer>();
        int[] pos = new int[2];
        for(int i = 0; i<nums.length;i++){
            if(hm.containsKey(target-nums[i])){
                pos[0] = hm.get(target-nums[i]);
                pos[1] = i;
                break;
            }else{
                hm.put(nums[i],i);
            }
            
        }
        return pos;
    }
}

Using a HashMap to keep tracking what number has been visited, O(n) solution