Question

Given a string, write an algorithm to find the character inside that appeared only once. For example, the result for string “abaccdeff” is b.

Solution

We can use a hashtable to count the appearance times for each character. Then rescan the hashtable for the character that appeared only once. These two step take O(n) each, therefore the overall time complexity is O(n).

```# function to find char appeared once
def find_appeared_once(string):
hashtable = {} # a hashtable

# loop for each char in string
for char in string:
if char not in hashtable:
hashtable[char] = 1
else:
hashtable[char] += 1

# find the item appeared only once
for char, count in hashtable.items():
if count == 1:
return char

# main
print find_appeared_once('abaccdeff')```

```#include <iostream.h>
#include <string.h>

char FirstNotRepeatingChar(char* pString)
{
if(!pString)
return 0;

const int tableSize = 256;
unsigned int hashTable[tableSize];
for(unsigned int i = 0; i < tableSize; ++ i)
hashTable[i] = 0;

char* pHashKey = pString;
while(*(pHashKey) != '/0')
hashTable[*(pHashKey++)] ++;

pHashKey = pString;
while(*pHashKey != '/0')
{
if(hashTable[*pHashKey] == 1)
return *pHashKey;

pHashKey++;
}

return *pHashKey;
}

int main()
{
cout<<"请输入一串字符："<<endl;
char s[100];
cin>>s;
char* ps=s;
cout<<FirstNotRepeatingChar(ps)<<endl;
return 0;
}

//////////////////////////////////////////

abaccdeff
b
Press any key to continue
///////////////////////////////////////////```