Complementary tutorial 2 – strings++

Prepared by: Valentin Kravtsov

Given a string, reverse it

char str[] = "abcdefghkl";

int i,str_len = strlen(str);

for(i=0 ; i<str_len/2 ; i++){

swap(str+i , str+str_len – 1 – i );

}

Output: lkhgfedcba

Given string of size n, rotate it m places to the right: No new arrays are allowed. O(nm)

abcdefghk => fghkabcde (n=9, m=4).

char tmp,str[] = "abcdefghk";

int i,j, m=4, n = strlen(str);

for(i=0;i<m;i++){

tmp = str[n-1];

for(j=n-2;j>=0;j--) { str[j+1] = str[j]; }

str[0] = tmp;

}

Output: fghkabcde

Given string of size n, rotate it m places to the right: (n=9, m=4). The efficient version. O(n)

abcdefghk => fghkabcde

The algorithm:

1. Identify the new first letter: “f”

2. Reverse both parts: edcbakhgf

3. Reverse the entire string: fghkabcde

The code:

void reverse(char* str, int from, int to){ int i, half_len = (to-from+1)/2; for(i=0;i<half_len;i++){ swap(str+from+i, str+to-i); } }

char str[] = "abcdefghk";int i, m=4, n = strlen(str), break_point = n-m; reverse(str,0,break_point-1);reverse(str,break_point,n-1);reverse(str,0,n-1);

Output: fghkabcde

Given string of with several words, reverse the order of words, (not characters), no additional arrays/strings are allowed. O(n)

ima aba and bamba => bamba and aba ima

The algorithm:

1. Reverse the whole string:

abmab dna aba ami

2. Reverse the letters in each word:

bamba and aba ima

Given string of with several words, reverse the order of words, (not characters), no additional arrays/strings are allowed.

ima aba and bamba => bamba and aba ima

char tmp,str[] = "ima aba and bamba";int end,start=0, n = strlen(str); reverse(str,0,n-1);while( find_next_word(str,&start,&end) ){ reverse(str,start,end); start=end+1; }

int find_next_word(char* str, int* start, int* end){

for( ; str[*start] && str[*start]==' '; (*start)++){}

if( !str[*start] ) return 0;

for(*end = *start ; str[*end+1] &&

str[*end+1] !=' ' ; (*end)++){}

return 1;

}

Finds next word by updating start and end to appropriate values. Return true/false (1/0) if a new word is found.

Compile/not compile questions

Compile/not compile questions

Compile/not compile questions

Compile/not compile questions

Given an array of characters of size n, and a new “order”, find if it is “newly ordered”.

Your very challenging homework…

Given an array of integers of size n, where one number appears more then n/2 times. Find this number, by scanning the array one time only.