Given an encoded string where repetitions of substrings are represented as substring followed by count of substrings. For example, if encrypted string is “ab2cd2” and k=4, so output will be ‘b’ because decrypted string is “ababcdcd” and 4th character is ‘b’.
Note: Frequency of encrypted substring can be of more than one digit. For example, in “ab12c3”, ab is repeated 12 times. No leading 0 is present in frequency of substring.
Input: "a2b2c3", k = 5 Output: c Decrypted string is "aabbccc" Input: "ab4c2ed3", k = 9 Output : c Decrypted string is "ababababccededed"
The solution discussed in previous post requires additional O(n) space. The following post discuss a solution that requires constant space. The stepwise algorithm is:
- Find length of current substring. Use two pointers. Fix one pointer at beginning of substring and move another pointer until a digit is not found.
- Find frequency of repetition by moving the second pointer further until an alphabet is not found.
- Find length of substring if it is repeated by multiplying frequency and its original length.
- If this length is less than k, then required character lies in substring that follows. Subtract this length from k to keep count of number of characters that are required to be covered.
- If length is less than or equal to k, then required character lies in current substring. As k is 1-indexed reduce it by 1 and then take its mod with original substring length. Required character is kth character from starting of substring which is pointed by first pointer.
Below is the implementation of the above approach:
// C# program to find K’th character in
// decrypted string
// Function to find K’th character in
// Encoded String
static char encodedChar(char str, int k)
int i, j;
int n = str.Length;
// To store length of substring
// To store length of substring when
// it is repeated
// To store frequency of substring
i = 0;
while (i < n)
j = i;
len = 0;
freq = 0;
// Find length of substring by
// traversing the string until
// no digit is found.
while (j < n && char.IsLetter(str[j]))
// Find frequency of preceding substring.
while (j < n && char.IsDigit(str[j]))
freq = freq * 10 + (str[j] - '0');
// Find length of substring when
// it is repeated.
num = freq * len;
// If length of repeated substring is less than
// k then required character is present in next
// substring. Subtract length of repeated
// substring from k to keep account of number of
// characters required to be visited.
if (k > num)
k -= num;
i = j;
// If length of repeated substring is
// more or equal to k then required
// character lies in current substring.
k %= len;
return str[i + k];
// This is for the case when there
// are no repetition in string.
// e.g. str=”abced”.
return str[k – 1];
// Driver Code
public static void Main(String args)
String str = “abced”;
int k = 4;
// This code is contributed by PrinciRaj1992
Time Complexity: O(n)
Auxiliary Space: O(1)
- Find k'th character of decrypted string | Set 1
- Find a string such that every character is lexicographically greater than its immediate next character
- Find the character in first string that is present at minimum index in second string
- Queries to find the last non-repeating character in the sub-string of a given string
- Queries to find the first non-repeating character in the sub-string of a string
- Find one extra character in a string
- Find last index of a character in a string
- Find the first repeated character in a string
- Given a string, find its first non-repeating character
- Find the last non repeating character in string
- Find repeated character present first in a string
- Program to find the kth character after decrypting a string
- Find i'th Index character in a binary string obtained after n iterations
- Efficiently find first repeated character in a string without using any additional data structure in one traversal
- Replace every character of string by character whose ASCII value is K times more than it
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