leetcode ZigZag Conversion

class Solution {
 public:
  string convert(string s, int nRows) {
    int i, j, flg;
    vector<char> vec;
    vector<vector<char>> zigzag(nRows,vec);
    for (i = 0, j = 0, flg = 1; i < s.length(); ++i) {
      zigzag[j].push_back(s[i]);
      if (flg && j == nRows - 1)
        flg = 0;
      else if (!flg && j == 0)
        flg = 1;
      if (nRows > 1)
        j = flg > 0 ? (j + 1):(j - 1);
    }
    string res = "";
    for (i = 0; i < nRows; ++i)
      for (j = 0; j < zigzag[i].size(); ++j)
        res += zigzag[i][j];
    return res;
  }
};

class Solution {
 public:
  string convert(string s, int nRows) {
    int len = s.length(), zigSize = nRows*2 - 2, i, j, tmp;
    string res = "";
    if (nRows <= 1) return s;
    for (i = 0; i < nRows; ++i) {
      for (j = i; j < len; j += zigSize) {
        res += s.substr(j,1);
        tmp = j + zigSize - 2 * i;
        if (i != 0 && i != nRows - 1 && tmp < len) {
          res += s.substr(tmp, 1);
        }
      }
    }
    return res;
  }
};