String Transforms Into Another String in Python

Sometimes we need to check if we can transform one string into another by converting all occurrences of specific characters. This problem involves character mapping and transformation rules to determine if str1 can be converted to str2.

In one conversion, we can convert all occurrences of one character in str1 to any other lowercase English character. The key constraint is that we need at least one "free" character (not used in str2) to perform intermediate conversions.

Problem Example

Given str1 = "aabcc" and str2 = "ccdee", we can transform str1 to str2 ?

• Convert 'c' to 'e': "aabee"

• Convert 'b' to 'd': "aadee"

• Convert 'a' to 'c': "ccdee"

Algorithm Approach

The solution uses two key concepts ?

1. Character Mapping: Each character in str1 must map to exactly one character in str2

2. Free Character Requirement: We need at least one unused character for intermediate transformations, unless str1 equals str2

Implementation

def can_convert(str1, str2):
    if len(str1) != len(str2):
        return False
    
    if str1 == str2:
        return True
    
    # Create mapping from str1 to str2
    char_map = {}
    for c1, c2 in zip(str1, str2):
        if c1 in char_map:
            if char_map[c1] != c2:
                return False
        else:
            char_map[c1] = c2
    
    # Check if we have a free character
    unique_chars_str2 = set(str2)
    return len(unique_chars_str2) < 26

# Test the function
print(can_convert("aabcc", "ccdee"))
print(can_convert("leetcode", "codeleet"))
print(can_convert("abcdefghijklmnopqrstuvwxyz", "bcdefghijklmnopqrstuvwxyza"))

True
False
False

How It Works

The algorithm follows these steps ?

1. Length Check: Both strings must have the same length

2. Identity Check: If strings are identical, no conversion is needed

3. Mapping Validation: Each character in str1 must consistently map to the same character in str2

4. Free Character Check: We need at least one character (out of 26) not used in str2 for intermediate conversions

Edge Cases

def can_convert(str1, str2):
    if len(str1) != len(str2):
        return False
    
    if str1 == str2:
        return True
    
    char_map = {}
    for c1, c2 in zip(str1, str2):
        if c1 in char_map:
            if char_map[c1] != c2:
                return False
        else:
            char_map[c1] = c2
    
    unique_chars_str2 = set(str2)
    return len(unique_chars_str2) < 26

# Edge case: All 26 characters used in str2
print(can_convert("a", "b"))  # True - has free characters
print(can_convert("aa", "bb"))  # True - can convert directly
print(can_convert("ab", "ba"))  # True - can use intermediate character

True
True
True

Key Points

• Consistent Mapping: Each character in str1 must always map to the same character in str2

• Free Character Rule: If all 26 lowercase letters appear in str2, transformation becomes impossible (except when str1 == str2)

• Order Independence: The actual order of transformations doesn't matter for the final result

Conclusion

To transform one string into another, ensure consistent character mapping and verify that at least one character remains unused in the target string. This free character enables intermediate conversions when needed.