0406MediumHash TableStringCounting

Restamps To Match The Tray

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Trains the technique from

LeetCode 1347Minimum Number of Steps to Make Two Strings Anagram

This is an original problem, written from a brief that listed the technique, the difficulty, the topics, the function shape and the input bounds — none of that problem's wording, examples, hints or editorials. The link is there so you can map your practice onto the standard set.

Same function shape, different story and different numbers.

A ceramics studio keeps two trays with the same number of tiles. The trays arrive as strings target and tray of equal length, and each character is the lowercase letter stamped on one tile.

You may take any tile out of tray and restamp it with any lowercase letter you like. One restamp changes one tile, and a restamped tile stays in tray.

Return the fewest restamps that leave tray holding exactly the same tally of every letter as target. Where the tiles sit inside a tray never matters, only how many of each letter it holds.

Examples

Example 1

Input: target = "kite", tray = "kits"
Output: 1

Both trays hold one `k`, one `i` and one `t`. The `s` in `tray` has no counterpart, so restamping it as `e` gives the tallies of `target`.

Example 2

Input: target = "sunny", tray = "runny"
Output: 1

The `u`, both `n`s and the `y` already line up in tally. Restamping the `r` as `s` finishes the job.

Example 3

Input: target = "wolf", tray = "flow"
Output: 0

Each of `w`, `o`, `l` and `f` appears once in both trays, so no restamp is needed even though the tiles sit in a different order.

Example 4

Input: target = "zzzz", tray = "zabc"
Output: 3

`target` needs four `z` tiles and `tray` has one, so the `a`, `b` and `c` all get restamped as `z`.

Example 5

Input: target = "moon", tray = "noon"
Output: 1

`tray` holds two `n` tiles where `target` holds one, and holds no `m` where `target` holds one, so restamping one `n` as `m` is enough.

Constraints

1 <= target.length <= 5 * 10^4
target.length == tray.length
target and tray consist of lowercase English letters only.

The signature

The editor is preloaded with this. It matches the parent problem's shape, so a solution that works here transfers to a judge unchanged.

Python

def min_steps(target: str, tray: str) -> int:

Java

public int minSteps(String target, String tray)

The intended approach

Time O(n) with a fixed 26-letter alphabetSpace O(1) - a fixed 26-slot tally

Tally how many tiles of each letter `target` wants, then walk `tray` once. A tile whose letter still has an unclaimed slot in the tally claims it and stays as it is; a tile whose letter has no slot left is one that has to be restamped. The count of unclaimed tiles is the answer, and no restamp ever needs to be undone because a restamped tile can be given whichever letter is short.

Pythonentry point: min_steps

def min_steps(target, tray):
    # Time O(n + 26): one pass to tally the target, one pass over the tray.
    # Space O(26), which is O(1) since the alphabet is fixed.
    spare = [0] * 26
    for ch in target:
        spare[ord(ch) - 97] += 1
    restamps = 0
    for ch in tray:
        slot = ord(ch) - 97
        if spare[slot] > 0:
            spare[slot] -= 1
        else:
            restamps += 1
    return restamps

The obvious version

Hold the target's tiles in a list and, for each tile of the tray, pull one matching tile out of the list if there is one; the tiles that found no match are the restamps.

This is not filler. It was written independently of the fast solution and the two were run against thousands of random inputs to confirm they agree — which is how the answer above is known to be right. Read it when the intended approach feels like a rabbit pulled out of a hat.

Python

def bf_min_steps(target, tray):
    pool = list(target)
    matched = 0
    for ch in tray:
        if ch in pool:
            pool.remove(ch)
            matched += 1
    return len(tray) - matched

How this problem was checked

Reference solution executed against 12 test cases — 12 passed
5 worked examples confirmed by running the code
0 eight-word sequences shared with the parent problem's text
Agrees with an independent brute-force implementation on random inputs
Test set proven to catch deliberately broken solutions
Reviewed by a separate quality pass
Function arity matches the parent problem's

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