【leetcode】Two Sum（unordered_map的使用）

1. Two Sum

leetcode题目描述

题目描述：

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.

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

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

题解：

找到两个数使其和为target，返回两个数的下标。
unordered_map<int, int>:
key:待查找数组中元素值
value：元素在数组中下标值

Solution1

/* O(n) by c++ */
class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        //Key is the number and value is its index in the vector.
        unordered_map<int, int> map;
        vector<int> result;
        int size = nums.size();
        for (int i = 0; i < size; i++) {
            int numtofind = target - nums[i];
            if (map.find(numtofind) != map.end()) {
                result.push_back(map[numtofind]);
                result.push_back(i);
            }
            map[nums[i]] = i; 
        }
        return result;
    }
};