Transform String to Another using Minimum Operations

Given two strings s1 and s2. Find the minimum number of steps required to transform string s1 into string s2. The only allowed operation for the transformation is selecting a character from string s1 and inserting it in the beginning of string s2.

Examples:

Input: s1 = "abd", s2 = "bad"
Output: 1
Explanation: The conversion can take place in 1 operation: Pick 'b' and place it at the front.

Input: s1 = "GeeksForGeeks" s2 = "ForGeeksGeeks"
Output: 3
Explanation: The conversion can take place in 3 operations:
Pick 'r' and place it at the front, A = "rGeeksFoGeeks"
Pick 'o' and place it at the front, A = "orGeeksFGeeks"
Pick 'F' and place it at the front, A = "ForGeeksGeeks"

Using Hash Map (or Dictionary) and Two Pointers - O(n) Time

The transformation is only possible if both strings have the same characters with the exact same frequencies. use a HashMap (or a frequency array) to count the characters in string s1 and then "cancel them out" using string s2.

If frequencies are same, then begin matching from end of both strings.

• Keep moving both pointers while there is a match
• For mismatches, increment result and move only pointer of s1 as the current substring of s1 can match with current prefix of s2.

3

Note : We can further optimize the above approach by using a count array of size 256 instead of map.