Swap Adjacent in LR String - Two Pointers [JS]

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Description

Solution: Two Pointers

Being able to swap "XL" with "LX" means we can move L as far LEFT as we want until it reaches an R.
Being able to swap RX" with "XR" means we can move R as far RIGHT as we want until it reaches an L.
When L and R meet, they can't be swapped.

Conditions that need to be met:

  1. Start without X's must be equal to end without X's.
  2. Positions of L in start must be <= to the positions of L in end.
  3. Positions of R in start must be >= to the positions of R in end.

To check for conditions 2 & 3, we can keep a running counter of L's and R's in start and end.
If the count of L's in start is greater than the count of L's in end, it is invalid.
If the count of R's in start is less than the count of R's in end, it is invalid.

Time Complexity: O(n) 79ms
Space Complexity: O(1) 44.3MB

var canTransform = function(start, end) {  
  let n = start.length;
  let start_str = "", end_str = "";
  for (let i = 0; i < n; i++) {
    if (start[i] !== 'X') start_str += start[i];
    if (end[i] !== 'X') end_str += end[i];
  }
  if (start_str !== end_str) return false; // the start and end without X's should be equal.

  let start_l_count = 0, end_l_count = 0;
  let start_r_count = 0, end_r_count = 0;
  for (let i = 0; i < n; i++) {
    if (start[i] === 'L') start_l_count++;
    else if (start[i] === 'R') start_r_count++;
    
    if (end[i] === 'L') end_l_count++;
    else if (end[i] === 'R') end_r_count++;
    
    if (start_l_count > end_l_count) return false; // this means there is an L in start ahead of an L in end.
    if (start_r_count < end_r_count) return false; // this means there is an R in start behind an R in end.
  }
  return true;
};

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Location: Auckland, New Zealand

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