<p>You are given two strings <code>start</code> and <code>target</code>, both of length <code>n</code>. Each string consists <strong>only</strong> of the characters <code>&#39;L&#39;</code>, <code>&#39;R&#39;</code>, and <code>&#39;_&#39;</code> where:</p>

<ul>
	<li>The characters <code>&#39;L&#39;</code> and <code>&#39;R&#39;</code> represent pieces, where a piece <code>&#39;L&#39;</code> can move to the <strong>left</strong> only if there is a <strong>blank</strong> space directly to its left, and a piece <code>&#39;R&#39;</code> can move to the <strong>right</strong> only if there is a <strong>blank</strong> space directly to its right.</li>
	<li>The character <code>&#39;_&#39;</code> represents a blank space that can be occupied by <strong>any</strong> of the <code>&#39;L&#39;</code> or <code>&#39;R&#39;</code> pieces.</li>
</ul>

<p>Return <code>true</code> <em>if it is possible to obtain the string</em> <code>target</code><em> by moving the pieces of the string </em><code>start</code><em> <strong>any</strong> number of times</em>. Otherwise, return <code>false</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> start = &quot;_L__R__R_&quot;, target = &quot;L______RR&quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> 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 &quot;<strong>L</strong>___R__R_&quot;.
- Move the last piece one step to the right, start becomes equal to &quot;L___R___<strong>R</strong>&quot;.
- Move the second piece three steps to the right, start becomes equal to &quot;L______<strong>R</strong>R&quot;.
Since it is possible to get the string target from start, we return true.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> start = &quot;R_L_&quot;, target = &quot;__LR&quot;
<strong>Output:</strong> false
<strong>Explanation:</strong> The &#39;R&#39; piece in the string start can move one step to the right to obtain &quot;_<strong>R</strong>L_&quot;.
After that, no pieces can move anymore, so it is impossible to obtain the string target from start.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> start = &quot;_R&quot;, target = &quot;R_&quot;
<strong>Output:</strong> false
<strong>Explanation:</strong> The piece in the string start can move only to the right, so it is impossible to obtain the string target from start.</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>n == start.length == target.length</code></li>
	<li><code>1 &lt;= n &lt;= 10<sup>5</sup></code></li>
	<li><code>start</code> and <code>target</code> consist of the characters <code>&#39;L&#39;</code>, <code>&#39;R&#39;</code>, and <code>&#39;_&#39;</code>.</li>
</ul>

class Solution:
    def canChange(self, start: str, target: str) -> bool:
        
        # two pointers, for swap position of char in target with char in start

        def swap(s, t, a, b):
            tmp = s[a]
            s[a] = t[a]
            s[b] = tmp
            return s

        def find_char(index, s):
            #print("find chars", s, index, char)
            while index < len(s) and s[index] == "_":
                index += 1
            
            return index

        start = list(start)
        middle = list(start)
        target = list(target)
        n = len(start)
        i, j = -1, -1
        # return early if mis-matched number of chars
        if sorted(start.copy()) != sorted(target.copy()):
            return False

        while True:
            # we've reached the end of each string
            if i == n and j == n:
                break
            
            i = find_char(i+1, start)
            j = find_char(j+1, target)

            if i == n or j == n:
                break

            # if they don't match, return false
            if start[i] != target[j]: break
            # since we are at "R", we can only move right. 
            # if i >j then there is no way we can get the result. 
            # Example. Start : _R  Target=R_  in this case i=1 > j=0
            if start[i] == "R" and i > j: break
            # since we are at "L", we can only move left. 
            # if i<j  then ther eis no way we can get the result. 
            # Example Start : L_ Target =_L . in this case i=0 < j=1
            if start[i] == "L" and i < j: break
            # for comparison
            swap(middle, target, i, j)
            #print("iteration", "".join(middle), i, j)


        middle = "".join(middle)
        target = "".join(target)
        print(middle, target)
        return middle == target
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