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Add C++ solution to two sum problem

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	`1`	`+#include <stdio.h>`
	`2`	`+#include <utility>`
	`3`	`+#include <unordered_map>`
	`4`	`+#include <vector>`
	`5`	`+#include <string>`
	`6`	`+`
	`7`	`+class Solution {`
	`8`	`+public:`
	`9`	`+ std::vector<int> twoSum(std::vector<int>& nums, int target) {`
	`10`	`+ // key: target - encountered num, value: index of encountered num`
	`11`	`+ std::unordered_map<int, int> num_pos;`
	`12`	`+ int num_items = nums.size();`
	`13`	`+`
	`14`	`+ for (int i=0; i < num_items; i++) {`
	`15`	`+ int curr_num = nums[i];`
	`16`	`+ // If the current number is not in the map`
	`17`	`+ if (num_pos.count(curr_num) == 0) {`
	`18`	`+ int sec_num = target-curr_num;`
	`19`	`+ num_pos.insert(std::make_pair(sec_num, i));`
	`20`	`+ }`
	`21`	`+ // If the current number is in the map, you found the answer`
	`22`	`+ else {`
	`23`	`+ return {num_pos[curr_num], i};`
	`24`	`+ }`
	`25`	`+ }`
	`26`	`+`
	`27`	`+ // If we go through the entire for loop without returning the answer`
	`28`	`+ // it means there was no valid pair`
	`29`	`+ return std::vector<int>();`
	`30`	`+ }`
	`31`	`+};`

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