The offer--of the Sword 38: Number of occurrences of the number in the sorted array (using two-point lookup to find the first and last position)

Topic:

Counts the number of occurrences of a number in a sorted array.

Ideas:

Because it is a well-ordered array, a binary lookup algorithm can be used.

Generally the most easily thought of the idea is to use a binary search to find a first, and then go to his left and right side of the traverse, but this method because in the N array may also have n K, so

The complexity of the lookup is still O (n)

You can first find the first occurrence by using the binary lookup algorithm, that is, when you find one, see if it is either k or whether it is the first one to find this paragraph.

Then the same two-point search to find the last occurrence of the position.

1#include <iostream>2#include <vector>3 using namespacestd;4 5 intCount_number (vector<int>& VEC,intN)6 {7     if(Vec.empty ())8         return 0;9     intlen=vec.size ();Ten     intRe=0; One     intI=0; A     intj=len-1; -  -     intre1=0; the      while(1) -     { -         if(i==j) -         { +             if(vec[i]==N) -{re1=i; Break;} +             Else A                 return 0; at         } -         intk= (j+i)/2; -  -         if(vec[k]==N) -         { -             if(k==i) in{re1=k; Break;} -             if(vec[k-1]!=N) to{re1=k; Break;} +j=k-1; -         } the         Else if(vec[k]<N) *         { $i=k+1;Panax Notoginseng         } -         Else thej=k-1; +     } A  thecout<<"re1="<<re1<<Endl; +  -     intRe2=0; $I=0; $j=len-1; -      while(1) -     { the         if(i==j) -         {Wuyi             if(vec[i]==N) the{re2=i; Break;} -             Else Wu                 return 0; -         } About         intk= (j+i)/2; $  -         if(vec[k]==N) -         { -             if(k==j) A{re2=k; Break;} +             if(vec[k+1]!=N) the{re2=k; Break;} -i=k+1; $         } the         Else if(vec[k]<N) the         { thei=k+1; the         } -         Else inj=k-1; the     } thecout<<"re2="<<re2<<Endl; Aboutre=re2-re1+1; the     returnre; the } the  + intMain () - { the     intary[8]={1,2,3,3,3,3,4,5};Bayivector<int> Vec (ary,ary+8); the  theCout<<count_number (VEC,3) <<Endl; -System"Pause"); -}

The offer--of the Sword 38: Number of occurrences of the number in the sorted array (using two-point lookup to find the first and last position)