Reverse the ordering of words in a string
I have this string s1 = "My name is X Y Z"
and I want to reverse the order of the words so that s1 = "Z Y X is name My"
.
I can do it using an additional array. I thought hard but is it possible to do it inplace (without using additional data structures) and with the time complexity being O(n)?
Solution 1:
Reverse the entire string, then reverse the letters of each individual word.
After the first pass the string will be
s1 = "Z Y X si eman yM"
and after the second pass it will be
s1 = "Z Y X is name My"
Solution 2:
reverse the string and then, in a second pass, reverse each word...
in c#, completely in-place without additional arrays:
static char[] ReverseAllWords(char[] in_text)
{
int lindex = 0;
int rindex = in_text.Length - 1;
if (rindex > 1)
{
//reverse complete phrase
in_text = ReverseString(in_text, 0, rindex);
//reverse each word in resultant reversed phrase
for (rindex = 0; rindex <= in_text.Length; rindex++)
{
if (rindex == in_text.Length || in_text[rindex] == ' ')
{
in_text = ReverseString(in_text, lindex, rindex - 1);
lindex = rindex + 1;
}
}
}
return in_text;
}
static char[] ReverseString(char[] intext, int lindex, int rindex)
{
char tempc;
while (lindex < rindex)
{
tempc = intext[lindex];
intext[lindex++] = intext[rindex];
intext[rindex--] = tempc;
}
return intext;
}
Solution 3:
Not exactly in place, but anyway: Python:
>>> a = "These pretzels are making me thirsty"
>>> " ".join(a.split()[::-1])
'thirsty me making are pretzels These'
Solution 4:
In Smalltalk:
'These pretzels are making me thirsty' subStrings reduce: [:a :b| b, ' ', a]
I know noone cares about Smalltalk, but it's so beautiful to me.
Solution 5:
You cannot do the reversal without at least some extra data structure. I think the smallest structure would be a single character as a buffer while you swap letters. It can still be considered "in place", but it's not completely "extra data structure free".
Below is code implementing what Bill the Lizard describes:
string words = "this is a test";
// Reverse the entire string
for(int i = 0; i < strlen(words) / 2; ++i) {
char temp = words[i];
words[i] = words[strlen(words) - i];
words[strlen(words) - i] = temp;
}
// Reverse each word
for(int i = 0; i < strlen(words); ++i) {
int wordstart = -1;
int wordend = -1;
if(words[i] != ' ') {
wordstart = i;
for(int j = wordstart; j < strlen(words); ++j) {
if(words[j] == ' ') {
wordend = j - 1;
break;
}
}
if(wordend == -1)
wordend = strlen(words);
for(int j = wordstart ; j <= (wordend + wordstart) / 2 ; ++j) {
char temp = words[j];
words[j] = words[wordend - (j - wordstart)];
words[wordend - (j - wordstart)] = temp;
}
i = wordend;
}
}