【C++ 算法笔记】 Sort的六种方法 【二】Quick Sort

IMPORTANT

THIS IS NOTE I MADE WHEN STUDY C++ ALGORITHM

THIS NOTE WOULD ONLY HELP U SPEND LESS TIME ON UNDERSTNDING THE ALGORITHM AND CODE IMPLEMENTATION

ALL WORDS IN " " ARE DIRECTLY REFERENCE

ALL CODE IMPLEMATATION ARE DIRECTLY REFERENCE

PLEASE CLICK THE LINK BELOW TO READ ORIGINAL BOOK AND INSTRUCTION

https://cathyatseneca.gitbook.io/data-strutures-and-algorithms/

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Quick Sort

Part A:

template <class TYPE>
void QuickSort(vector<TYPE>& arr){
  QuickSort(arr,0,arr.size()-1);
}

Part B:

template <class TYPE>
void QuickSort(vector<TYPE>& arr, int left, int right){
  if(right-left <=3){
    InsertionSort(arr,left,right);
  }
  else{
    int pivotpt=right;
    int i=left;
    int j=right-1;
    TYPE pivot=arr[pivotpt];
    while(i<j){
      while(arr[i]<pivot) i++;
      while(arr[j]>pivot) j--;
      if(i<j)
        swap(arr[i++],arr[j--]);
    }
    swap(arr[i],arr[pivotpt]);
    QuickSort(arr,left,i-1);
    QuickSort(arr,i+1,right);
  }
}

• sort with “jumping”
• run time depends on different situation:
  • the best situation: O(nlogn)
  • the worst situation is: O(n^2)
• (index might change if there are some data with same value)?
• Recursice concept is needed

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TO implement this, a pivot point is needed.
PArt A create a new prototype for the function in partB.
In part B,
A pivot point is decleared with the most right value in the array.
In the first while-loop ensures i and j will meet each other,
and this is exact location of pivot point;
the following two while-loop finds the number smaller than pivot and larger than pivot respectively;
when those two numbers are finded,
switch their positions and continue.
Placed the pivot point in correct location represent the end of the firsr round,
repeat above step recursively by dividing this array into two pieces,
pivot uses to indetify the break.