2337. Move Pieces to Obtain a String

2337. Move Pieces to Obtain a String

Difficulty: Medium

Topics: Two Pointers, String

You are given two strings start and target, both of length n. Each string consists only of the characters 'L', 'R', and '_' where:

• The characters 'L' and 'R' represent pieces, where a piece 'L' can move to the left only if there is a blank space directly to its left, and a piece 'R' can move to the right only if there is a blank space directly to its right.


• The character '_' represents a blank space that can be occupied by any of the 'L' or 'R' pieces.

Return true if it is possible to obtain the string target by moving the pieces of the string start any number of times. Otherwise, return false.

Example 1:

• 
  Input: start = "LRR", target = "L______RR"


• 
  Output: true


• 
  Explanation: We can obtain the string target from start by doing the following moves:
  
  
  
  
  • Move the first piece one step to the left, start becomes equal to "L__RR".
  
  
  • Move the last piece one step to the right, start becomes equal to "L_R_R".
  
  
  • Move the second piece three steps to the right, start becomes equal to "L______RR".
  
  
  • Since it is possible to get the string target from start, we return true.
  
  
  
  
  
  

Example 2:

• 
  Input: start = "R_L_", target = "__LR"


• 
  Output: false


• 
  Explanation: The 'R' piece in the string start can move one step to the right to obtain "RL".
  
  
  
  
  • After that, no pieces can move anymore, so it is impossible to obtain the string target from start.
  
  
  
  
  
  

Example 3:

• 
  Input: start = "R", target = "R"


• 
  Output: false


• 
  Output: The piece in the string start can move only to the right, so it is impossible to obtain the string target from start.

Constraints:

• n == start.length == target.length


• 1 <= n <= 105


• 
  start and target consist of the characters 'L', 'R', and '_'.

Hint:

1. After some sequence of moves, can the order of the pieces change?


2. Try to match each piece in s with a piece in e.

Solution:

We need to check if we can transform the string start into the string target by moving pieces ('L' and 'R') as per the given rules. The main constraints to consider are:

• 'L' can only move left (and cannot cross other 'L' or 'R' pieces).


• 'R' can only move right (and cannot cross other 'L' or 'R' pieces).


• We can use any blank spaces ('_') to move pieces around.

Plan:

1. Length Check: First, check if both strings have the same length. If they don't, return false immediately.




2. Character Frequency Check: The number of 'L's, 'R's, and '_' in the start string must match the respective counts in the target string because the pieces cannot be duplicated or destroyed, only moved.



3. 
   

   Piece Matching Using Two Pointers:

   
   
   
   
   
   • Traverse through both strings (start and target).
   
   
   • Check if each piece ('L' or 'R') in start can move to its corresponding position in target.
   
   
   • Specifically:
     
     
     
     
     • 'L' should always move to the left (i.e., it must not be in a position where a piece in target should move right).
     
     
     • 'R' should always move to the right (i.e., it must not be in a position where a piece in target should move left).
     
     
     
     
   
   
   
   

Algorithm Explanation:

1. 
   

   Filter L and R Positions:

   
   
   
   
   
   • Remove all _ from both strings start and target to compare the sequence of L and R. If the sequences differ, return false immediately.
   
   
   
   


2. 
   

   Two-Pointer Traversal:

   
   
   
   
   
   • Use two pointers to iterate over the characters in start and target.
   
   
   • Ensure that:
     
     
     
     
     • For L, the piece in start can only move left, so its position in start should be greater than or equal to its position in target.
     
     
     • For R, the piece in start can only move right, so its position in start should be less than or equal to its position in target.
     
     
     
     
   
   
   
   


3. 
   

   Output Result:

   
   
   
   
   
   • If all conditions are satisfied during traversal, return true. Otherwise, return false.
   
   
   
   

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