Two strings s and t are isomorphic if the characters in s can be replaced to get t. Every occurrence of a character must map to the same character, and no two characters may map to the same character (a one-to-one mapping). Return false if either string is null or the lengths differ.
Example 1:
Input: s = 'egg', t = 'add'
Output: true
Explanation: e maps to a, g maps to d. The mapping is consistent throughout.
Example 2:
Input: s = 'foo', t = 'bar'
Output: false
Explanation: o would need to map to both a and r, which is not allowed.
Example 3:
Input: s = 'paper', t = 'title'
Output: true
Explanation: p maps to t, a maps to i, e maps to l, r maps to e. Consistent one-to-one mapping.
Example 4:
Input: s = 'ab', t = 'a'
Output: false
Explanation: Different lengths are never isomorphic.
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