Sort Target String by Custom Order

Onsite coding: given a `target` string and an `order` string, return a permutation of `target` so that characters that appear in `order` are emitted in `order`'s sequence. Characters not in `order` may go anywhere. Equivalent to LeetCode 791 (Custom Sort String). Requirements Inputs: target (the string whose characters need to be rearranged), order (a permutation defining a custom alphabet ordering). Output: any permutation of target such that for any two characters x, y both present in order, x appears before y in the output iff x appears before y in order. Characters in target that are not present in order may appear anywhere in the result. Notes Standard solution: counter over target, walk order left to right, emit counter[c] copies of each c, then append the remaining characters (those not in order) in any order. Complexity O(|target| + |order|). Implement carefully when target may contain repeats not present in order — they go in a separate 'leftover' pass. An equivalent solution sorts target with a key that maps each character to its position in order (or len(order) for absent characters). Slightly slower (O(|target| log |target|)) and only worth mentioning as a baseline. Common micro-bug: forgetting to zero out the counter entries that have already been consumed, then double-emitting in the leftover pass. Preparation Practice writing the counter + walk solution in under 8 minutes. Stretch follow-up: rank characters in target by their order index and report the resulting permutation's longest-increasing-subsequence — common L5 add-on probe. For an interview where order is large and target is small, flip the algorithm: index order into pos[char], then sort target characters by pos.get(c, len(order)).