（原）通过合并排序算法加深对边界值（等号的取舍）的认识

 

#include <iostream>
#include <stdlib.h>
#include <time.h>
using namespace std;

void Merge(int a[], int left, int mid, int right, int* tempA, int* tempB)
{
    int lengthA = mid - left + 1, lengthB = right - mid;
    int i,j,k;
    for(i=0; i<lengthA; i++)
         tempA[i] = a[left+i];
       for(j=0; j<lengthB; j++)
           tempB[j] = a[mid+1+j];
    i = j = 0;
    k = left;
    while(i < lengthA && j < lengthB)
    {
        if(tempA[i] <= tempB[j])
            a[k++] = tempA[i++];
        else
            a[k++] = tempB[j++];
    }

    if(i < lengthA)
    {
        while(i < lengthA)
            a[k++] = tempA[i++];
    }

    if(j < lengthB)
    {
        while(j < lengthB)
            a[k++] = tempB[j++];
    }
    cout<<"After Merge: "<<left<<"  "<<mid<<"  "<<right<<endl;
    cout<<"|  ";
    for(int m=left; m<=right; m++)
        cout<<a[m]<<"  ";
    cout<<" |"<<endl;
    cout<<endl;
}

void _MergeSort(int* a, int lo, int hi, int* tempA, int* tempB)
{
    cout<<lo<<"  "<<hi<<endl;
    if(lo < hi)
      {
           int middle = (hi + lo)/2;
           _MergeSort(a, lo, middle, tempA, tempB);
         _MergeSort(a, middle+1, hi, tempA, tempB);
         Merge(a, lo, middle, hi, tempA, tempB);
      }
}

void MergeSort(int* a, int length)
{
    int* tempA = new int[length];
    int* tempB = new int[length];
    _MergeSort(a, 0, length-1, tempA, tempB);
    delete []tempA;
    delete []tempB;
}

int main()
{
    int i,n;
    cin>>n;
    int* a =new int[n];
    srand(time(NULL));
    for(i=0; i<n; i++)
    {
        a[i] = rand()%100 + 1;
    }

    cout<<"Before MergeSort : "<<endl;
    for(i=0; i<n; i++)
         cout<<a[i]<<" ";
    cout<<endl;

    MergeSort(a, n);

    cout<<endl
        <<"After MergeSort : "<<endl;
    for(i=0; i<n; i++)
         cout<<a[i]<<" ";

    delete []a;
    return 0;
}

代码如上，当时由于while和if里面的条件没写好，导致数组越界。我总结了一个小技巧，就是用临界值的左边一个和右边一个值去做测试，例如这句

while(i < lengthA && j < lengthB)

当 i=lengthA-1 的时候可以继续执行吗？当 i=lengthA 的时候又可以继续执行吗？根据不同的情况决定是否应该使用等号。