C++:从头开始实现归并排序

Question

我是在自考，想写mergesort，没有实际上网查任何代码，并且在一定时间段内完成。我被困在这一点上，我不能简单地理解我做错了什么，因为据我所知，合并排序是将字符串划分到字符串只有 1 个字符的位置，然后再将它们合并回一起。我在下面编写的代码尝试 做同样的事情。我想知道是我弄错了概念，还是只是我的实现？

string merge(string str1, string str2) {
  string final = "";
  int i = 0, j = 0;
  bool fromStr1 = false;

  while(true) {
    if(str1[i] < str2[j]) {
      final += str1[i];
      i++;
      if(i == str1.size()) {
        break;
      }
    }
    else {
      final += str2[j];
      j++;
      if(j == str2.size()) {
        break;
        fromStr1 = true;
      }
    }
  }

  if(fromStr1) {
    for(int t = i; t < str1.size(); t++) {
      final += str1[t];
    }
  }
  else {
    for(int t = j; t < str2.size(); t++) {
      final += str2[t];
    }
  }

  return final;
}

string mergeSort(string str1, int start, int end) {
  if(end - start == 1)
    return str1;
  else {
    int pivot = (end - start) / 2;
    string newStr1 = mergeSort(str1, start, pivot);
    string newStr2 = mergeSort(str1, pivot + 1, end);
    return merge(newStr1, newStr2);
  }
}

Answer 1

注意变化:

#include <iostream>
#include <string>

using namespace std;

string merge(string str1, string str2) {
  string final = "";
  int i = 0, j = 0;
  bool fromStr1 = false;

  while (true) {
    if (i >= (int)str1.size()) {
      break;
    }
    if (j >= (int)str2.size()) {
      fromStr1 = true; // changed the order of this with break!
      break;
    }

    if (str1[i] < str2[j]) {
      final += str1[i];
      i++;
    }
    else {
      final += str2[j];
      j++;
    }
  }

  if (fromStr1) {
    for (int t = i; t < (int)str1.size(); t++) {
      final += str1[t];
    }
  }
  else {
    for(int t = j; t < (int)str2.size(); t++) {
      final += str2[t];
    }
  }

  return final;
}

string mergeSort(string str1) {
  int len = str1.size();
  if (len <= 1)
    return str1;
  else {
    string newStr1 = mergeSort(str1.substr(0, len / 2));
    string newStr2 = mergeSort(str1.substr(len / 2, len - len / 2));
    return merge(newStr1, newStr2);
  }
}

int main()
{
  cout << '"' << mergeSort("") << '"' << endl;
  cout << '"' << mergeSort("a") << '"' << endl;
  cout << '"' << mergeSort("ba") << '"' << endl;
  cout << '"' << mergeSort("132") << '"' << endl;
  cout << '"' << mergeSort("4321") << '"' << endl;
  cout << '"' << mergeSort("54321") << '"' << endl;
  return 0;
}

输出(ideone):

""
"a"
"ab"
"123"
"1234"
"12345"