寻找字符串中遗失的数字并修复

//example:Str="4142434546" Findout missing no 44.Add it to str;
//Output:"414243444546". 

#include <iostream>
#include <stdlib.h>

bool FixStr(const string& str, string* result) {
  if (str.empty()) return false;
  int len = str.length();
  int first_len = 1;
  for (; first_len <= len / 2; ++first_len) {
    int pos = 0;
    int num_len = first_len;
    bool find = false;
    bool matched = true;
    int num1 = atoi(str.substr(pos, pos + num_len)).c_str();
    pos = pos + num_len;
    int miss_pos = 0;
    int miss_num = 0;
    while (pos < len) {
      int num2_a = -1; 
      int num2_b = -1; 
      if (pos + num_len > len) {
        matched = false;
        break;
      } else {
        num2_a = atoi(str.substr(pos + num_len, pos + num_len).c_str());
      }   
      if (pos + num_len + 1 <= len) {
        num2_b = atoi(str.substr(pos + num_len, pos + num_len + 1).c_str());
      }   
      if (num1 == num2_a - 1) {
        num1 = num2_a;
        pos += num_len;
        continue;
      }   
      if (num1 == num2_b - 1) {
        num1 = num2_b;
        num_len++;
        pos += num_len;
        continue;
      }   
      if (find) {
        matched = false;
        break;
      } else {
        if (num1 == num2_a - 2 || num1 == num2_b - 2) {
          num1 += 2;
          find = true;
          miss_pos = pos;
          miss_num = num1 + 2;
          if (num1 == num2_b - 2) num_len++;
          pos += num_len;
          continue;
        }   
        matched = false;
        break;
      }   
    }   
    if (matched) {
      if (find) *result = str;
      else *result = str.substr(0, miss_pos).append(itoa(miss_num)).append(miss_pos, len);
      return true;
    }   
  }
  return false;
}