Reformat The String

Reformat The String

What is this problem about?

The Reformat The String problem asks you to interleave digits and letters from a string, alternating between the two types. If it's impossible (their counts differ by more than 1), return "". This easy string problem tests careful interleaving with size-based ordering. The string interview pattern is demonstrated.

Why is this asked in interviews?

Microsoft asks this to test string partitioning and interleaving — a common string processing task. The key constraint is that one group can be at most 1 element larger, determining which starts first.

Algorithmic pattern used

Separate + interleave. Separate digits into one list and letters into another. If |len(digits) - len(letters)| > 1, return "". Otherwise: let the larger group go first. Interleave by alternating between the two lists.

Example explanation

s="a0b1c2". Digits=[0,1,2], Letters=[a,b,c]. Counts equal. Interleave (either starts): "a0b1c2" or "0a1b2c". Return either valid arrangement: "a0b1c2".

s="leetcode1". Digits=[1], Letters=[l,e,e,t,c,o,d,e]. |8-1|=7 > 1 → return "".

Common mistakes candidates make

• Returning "" when counts differ by exactly 1 (allowed if larger goes first).
• Not building both groups before interleaving.
• Joining characters in wrong order (must strictly alternate).
• Off-by-one in the feasibility check.

Interview preparation tip

Interleaving problems with two groups check if the size difference allows alternation (at most 1 difference). The larger group always starts first. This same principle appears in "task scheduler" (alternate task types) and "organize string with k distance." Practice interleaving patterns: always separate → determine starting group → merge alternately.