1.two-sum.cpp

/*
 * @lc app=leetcode id=1 lang=cpp
 *
 * [1] Two Sum
 *
 * https://leetcode.com/problems/two-sum/description/
 *
 * algorithms
 * Easy (44.37%)
 * Likes:    11371
 * Dislikes: 391
 * Total Accepted:    2M
 * Total Submissions: 4.5M
 * Testcase Example:  '[2,7,11,15]\n9'
 *
 * 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].
 * 
 * 
 */
class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> ret;
        map<int, int> map;
        for (int i = 0; i < nums.size(); i++) {
            if (!map.count(nums[i])) {
                map[nums[i]] = i;
            }
            if (map.count(target - nums[i])) {
                int j = map[target - nums[i]];
                if (j < i) {
                    ret.push_back(j);
                    ret.push_back(i);
                    return ret;
                }
            }
        }
        return ret;
    }
};