text: // Hi, I'm Yanzhan. For more algothmic problems, visit my Youtube Channel (Yanzhan Yang's Youtube Channel) : https://www.youtube.com/channel/UCDkz-__gl3frqLexukpG0DA?view_as=subscriber or my Twitter Account (Yanzhan Yang's Twitter) : https://twitter.com/YangYanzhan or my GitHub HomePage (Yanzhan Yang's GitHub HomePage) : https://yanzhan.site .
  // For this specific algothmic problem, visit my Youtube Video :  .
  // It's fascinating to solve algothmic problems, follow Yanzhan to learn more!
  // Blog URL for this problem: https://yanzhan.site/interviewbit/first-missing-integer.html .
  
  int Solution::firstMissingPositive(vector<int> &A) {
      sort(A.begin(), A.end());
      int pre = -1, count = 0;
      for (int i = 0; i < A.size(); i++) {
          int now = A[i];
          if (now > 0) {
              if (now != pre) {
                  count++;
                  if (count != now) {
                      return count;
                  }
              }
          }
      }
      return count + 1;
  }
  