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Topic: puzzle: find first non repeated character (Read 2904 times) |
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puzzlecracker
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puzzle: find first non repeated character
« on: Dec 9th, 2004, 5:48pm » |
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Given an array of characters in array find first non-repeated character. ex. in "total" is 'o'
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Grimbal
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Re: puzzle: find first non repeated character
« Reply #1 on: Dec 10th, 2004, 5:57am » |
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Create an array of flags, one per character, set them to false.
Scan the string from the end. For each character: if the flag is false save the character and set the flag to true.
When done, the last saved character is the first non-repeated.
Advantage: for large strings, it is still O(n).
Problem: for short strings and large character sets, you still have to initialize the whole array of flags.
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puzzlecracker
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Re: puzzle: find first non repeated character
« Reply #2 on: Dec 10th, 2004, 9:34am » |
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where are you going to save the characters? create another array?
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Grimbal
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Re: puzzle: find first non repeated character
« Reply #3 on: Dec 14th, 2004, 3:35am » |
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yes. If the character set is fixed, it is O(1).
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Margit
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Re: puzzle: find first non repeated character
« Reply #4 on: Dec 16th, 2004, 12:47am » |
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How does that work with eg. "AAB" ?
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puzzlecracker
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Re: puzzle: find first non repeated character
« Reply #5 on: Dec 16th, 2004, 2:14pm » |
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Hi I just wrote the alg based on your psuedo code, but it doesnt work:
#include<iostream>
using namespace std;
int main()
{
bool charFlag[26];
char chars[26], *in="total";
int last=-1;
for(int i=0;i<26;charFlag[i]=false,i++); // clear the flag arr
for(int i=0;i<=sizeof(in);i++)
{
if(charFlag[in[i]-'a']==true)
continue;
charFlag[in[i]-'a']=true;
chars[++last]=in[i];
}
cout<<chars[last]<<endl;
system("PAUSE");
return 0;
}
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puzzlecracker
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Re: puzzle: find first non repeated character
« Reply #6 on: Dec 16th, 2004, 2:17pm » |
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Hi I just wrote the alg based on your psuedo code, but it doesnt work: (playing with code funcs  )
Code:
#include<iostream>
using namespace std;
int main()
{
bool charFlag[26];
char chars[26], *in="total";
int last=-1;
for(int i=0;i<26;charFlag[i]=false,i++); // clear the flag arr
for(int i=0;i<=sizeof(in);i++)
{
if(charFlag[in[i]-'a']==true)
continue;
charFlag[in[i]-'a']=true;
chars[++last]=in[i];
}
cout<<chars[last]<<endl;
system("PAUSE");
return 0;
} |
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towr
wu::riddles Moderator
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Re: puzzle: find first non repeated character
« Reply #7 on: Dec 16th, 2004, 2:35pm » |
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I think you missed the 'from the end' part.
Start at the end of the string, and scan to the front.
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puzzlecracker
Senior Riddler
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Re: puzzle: find first non repeated character
« Reply #8 on: Dec 16th, 2004, 3:24pm » |
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Sorry, forgot to traverse in reverse... the problem is actually valid of aab... it iwll report a is a last stored char.....
Code:i
#nclude<iostream>
using namespace std;
int main()
{
bool charFlag[26];
char chars[26], *in="aab";
int last=-1;
for(int i=0;i<26;charFlag[i]=false,i++); // clear the flag arr
for(int i=sizeof(in)+1;i>=0;i--)
{
if(charFlag[in[i]-'a'])
continue;
charFlag[in[i]-'a']=true;
last =i;
}
cout<<in[last]<<endl;
system("PAUSE");
return 0;
}
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puzzlecracker
Senior Riddler
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Re: puzzle: find first non repeated character
« Reply #9 on: Dec 16th, 2004, 5:45pm » |
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oK sorry for the bombardment: here is the correct solution to the problem
create a hashtable for each char,
zero out every element
for every occurance of the character, increment the coutner
go through string array again and if that char hashes to 1, return it.
i implemented hash using array
Code:
#include<iostream>
using namespace std;
int main()
{
int charFlag[26];
char *in="aab";
for(int i=0;i<26;charFlag[i]=0,i++); // clear the flag arr
for(int i=0;i<=sizeof(in);++i)
charFlag[in[i]-'a']++;
for(int i=0;i<=sizeof(in);++i)
if(charFlag[in[i]-'a']==1){
cout<<in[i]<<endl;
break;
}
system("PAUSE");
return 0;
}
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