Two Sum(两数之和) – 每天一道算法题

题目 #

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]

题目大意 #

在数组中找到 2 个数之和等于给定值的数字,结果返回 2 个数字在数组中的下标。

解题思路 #

这道题最优的做法时间复杂度是 O(n)。

顺序扫描数组,对每一个元素,在 map 中找能组合给定值的另一半数字,如果找到了,直接返回 2 个数字的下标即可。如果找不到,就把这个数字存入 map 中,等待扫到“另一半”数字的时候,再取出来返回结果。

java代码 #

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

golang代码 #

package leetcode

func twoSum(nums []int, target int) []int {
	m := make(map[int]int)
	for i := 0; i < len(nums); i++ {
		another := target - nums[i]
		if _, ok := m[another]; ok {
			return []int{m[another], i}
		}
		m[nums[i]] = i
	}
	return nil
}

 

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