844. Backspace String Compare

Problem Link

Description

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Given two strings s and t, return true if they are equal when both are typed into empty text editors. '#' means a backspace character.

Note that after backspacing an empty text, the text will continue empty.

 

Example 1:

Input: s = "ab#c", t = "ad#c"
Output: true
Explanation: Both s and t become "ac".

Example 2:

Input: s = "ab##", t = "c#d#"
Output: true
Explanation: Both s and t become "".

Example 3:

Input: s = "a#c", t = "b"
Output: false
Explanation: s becomes "c" while t becomes "b".

 

Constraints:

• 1 <= s.length, t.length <= 200
• s and t only contain lowercase letters and '#' characters.

 

Follow up: Can you solve it in O(n) time and O(1) space?

Solution

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Python3

class Solution:
    def backspaceCompare(self, s: str, t: str) -> bool:
        M, N = len(s), len(t)
        i, j = M - 1, N - 1
 
        while True:
            delCount = 0
            while i >= 0 and (delCount > 0 or s[i] == "#"):
                if s[i] == '#':
                    delCount += 1
                else:
                    delCount -= 1
                i -= 1
            
            delCount = 0
            while j >= 0 and (delCount > 0 or t[j] == "#"):
                if t[j] == '#':
                    delCount += 1
                else:
                    delCount -= 1
                j -= 1
            
            if i >= 0 and j >= 0 and s[i] == t[j]:
                i -= 1
                j -= 1
            else:
                break
        
        return i == -1 and j == -1