Input: A = [2], B = [1,3]
Output: [2,3]

Example 4:

Input: A = [1,2,5], B = [2,4]
Output: [5,4]

Note:

• 1 <= A.length <= 10000
• 1 <= B.length <= 10000
• 1 <= A[i] <= 100000
• 1 <= B[i] <= 100000
• It is guaranteed that Alice and Bob have different total amounts of candy.
• It is guaranteed there exists an answer.

 

Solution: HashTable

Time complexity: O(A+B)

Space complexity: O(B)

Clean version

// Author: Huahua
// Running time: 132 ms
class Solution {
public:
  vector<int> fairCandySwap(vector<int>& A, vector<int>& B) {    
    int sum_a = accumulate(begin(A), end(A), 0);
    int sum_b = accumulate(begin(B), end(B), 0);
    unordered_set<int> s(begin(B), end(B));    
    int diff = (sum_b - sum_a) / 2;
    for (int a : A)
      if (s.count(a + diff)) return {a, a + diff};
  }
};

Faster version

// Author: Huahua
// Running time: 68 ms
class Solution {
public:
  vector<int> fairCandySwap(vector<int>& A, vector<int>& B) {    
    int sum_a = accumulate(begin(A), end(A), 0);
    int sum_b = 0;
    unordered_set<int> s;
    for (int b : B) {
      sum_b += b;
      s.insert(b);
    }
    int diff = (sum_b - sum_a) / 2;
    for (int a : A)
      if (s.count(a + diff)) return {a, a + diff};
  }
};