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LPS in O(n) #41

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merged 2 commits into from
Oct 31, 2019
Merged

LPS in O(n) #41

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70f4222
Create fps_O(n)
Aashna-Agrawal Oct 31, 2019
3acac2e
Create LPS_O(n)
Aashna-Agrawal Oct 31, 2019
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54 changes: 54 additions & 0 deletions Algorithms/Array/LPS_O(n)
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Original file line number Diff line number Diff line change
@@ -0,0 +1,54 @@
//Returns Longest Palindrome Subsequence length.
//Time Complexity O(n).
int LPSManchers(string input) {
char forOddPalindrome = '-';
//Uncomment the next line for odd length palindromes only.
//forOddPalindrome = '$';

string newInput = "$";
for(int i = 0; i < input.length(); i++) {
newInput += input[i];
newInput += '$';
}
input = newInput;

int length = input.length();
int T[length];
for(int i = 0; i < length; i++) {
T[i] = 0;
}
int start = 0;
int end = 0;
for(int i = 0; i < length; ) {
while((start > 0) && (end < length - 1) && (input[start-1] == input[end+1])) {
start--;
end++;
}
T[i] = end - start + 1;
if(end == length - 1) {
break;
}
int newCenter = end + (i%2 == 0 ? 1 : 0);
for(int j = i + 1; j <= end; j++) {
T[j] = min(T[i - (j - i)], 2 * (end - j) + 1);
if(j + T[i - (j - i)]/2 == end) {
newCenter = j;
break;
}
}
i = newCenter;
end = i + T[i]/2;
start = i - T[i]/2;
}
int max = INT_MIN;
for(int i = 0; i < length; i++) {
if(input[i] != forOddPalindrome) {
int val;
val = T[i]/2;
if(max < val) {
max = val;
}
}
}
return max;
}
54 changes: 54 additions & 0 deletions Algorithms/Array/fps_O(n)
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Original file line number Diff line number Diff line change
@@ -0,0 +1,54 @@
//Returns Longest Palindrome Subsequence length.
//Time Complexity O(n).
int LPSManchers(string input) {
char forOddPalindrome = '-';
//Uncomment the next line for odd length palindromes only.
//forOddPalindrome = '$';

string newInput = "$";
for(int i = 0; i < input.length(); i++) {
newInput += input[i];
newInput += '$';
}
input = newInput;

int length = input.length();
int T[length];
for(int i = 0; i < length; i++) {
T[i] = 0;
}
int start = 0;
int end = 0;
for(int i = 0; i < length; ) {
while((start > 0) && (end < length - 1) && (input[start-1] == input[end+1])) {
start--;
end++;
}
T[i] = end - start + 1;
if(end == length - 1) {
break;
}
int newCenter = end + (i%2 == 0 ? 1 : 0);
for(int j = i + 1; j <= end; j++) {
T[j] = min(T[i - (j - i)], 2 * (end - j) + 1);
if(j + T[i - (j - i)]/2 == end) {
newCenter = j;
break;
}
}
i = newCenter;
end = i + T[i]/2;
start = i - T[i]/2;
}
int max = INT_MIN;
for(int i = 0; i < length; i++) {
if(input[i] != forOddPalindrome) {
int val;
val = T[i]/2;
if(max < val) {
max = val;
}
}
}
return max;
}