/*

//

// The implementation as below is bit tricky. but not difficult to understand

//

// 1) Traverse the array one by one

// 2) just put the `target - num[i]`(not `num[i]`) into the map

// so, when we checking the next num[i], if we found it existed in the map,

// it means we found the second one.

//

vector<int> twoSum(vector<int> &numbers, int target) {

unordered_map<int, int> m;

vector<int> result;

for(int i=0; i<numbers.size(); i++){

// not found the second one

if (m.find(numbers[i])==m.end() ) {

// store the first one position into the second one's key

m[target - numbers[i]] = i;

}else {

// found the second one

result.push_back(m[numbers[i]]+1);

result.push_back(i+1);

break;

}

}

return result;

}

// we also can store nums[i] into map, and find target - nums[i]

vector<int> twoSum(vector<int>& nums, int target) {

unordered_map<int, int> m;

vector<int> result;

for (int i=0; i<nums.size(); i++) {

if ( m.find(target - nums[i]) == m.end() ) {

m[nums[i]] = i;

}else{

result.push_back(m[target - nums[i]]);

result.push_back(i);

}

}

return result;

}