LeetCode #3668 — EASY

Restore Finishing Order

Build confidence with an intuition-first walkthrough focused on array fundamentals.

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The Problem

Problem Statement

You are given an integer array order of length n and an integer array friends.

order contains every integer from 1 to n exactly once, representing the IDs of the participants of a race in their finishing order.
friends contains the IDs of your friends in the race sorted in strictly increasing order. Each ID in friends is guaranteed to appear in the order array.

Return an array containing your friends' IDs in their finishing order.

Example 1:

Input: order = [3,1,2,5,4], friends = [1,3,4]

Output: [3,1,4]

Explanation:

The finishing order is [3, 1, 2, 5, 4]. Therefore, the finishing order of your friends is [3, 1, 4].

Example 2:

Input: order = [1,4,5,3,2], friends = [2,5]

Output: [5,2]

Explanation:

The finishing order is [1, 4, 5, 3, 2]. Therefore, the finishing order of your friends is [5, 2].

Constraints:

1 <= n == order.length <= 100
order contains every integer from 1 to n exactly once
1 <= friends.length <= min(8, n)
1 <= friends[i] <= n
friends is strictly increasing

Step 01

Brute Force Baseline

Problem summary: You are given an integer array order of length n and an integer array friends. order contains every integer from 1 to n exactly once, representing the IDs of the participants of a race in their finishing order. friends contains the IDs of your friends in the race sorted in strictly increasing order. Each ID in friends is guaranteed to appear in the order array. Return an array containing your friends' IDs in their finishing order.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Array · Hash Map

Example 1

[3,1,2,5,4]
[1,3,4]

Example 2

[1,4,5,3,2]
[2,5]

Step 02

Core Insight

What unlocks the optimal approach

Use a hash set for quick friend lookups from <code>friends</code>.
Iterate over <code>order</code>, checking the set to collect your friends in finishing order.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #3668: Restore Finishing Order
class Solution {
    public int[] recoverOrder(int[] order, int[] friends) {
        int n = order.length;
        int[] d = new int[n + 1];
        for (int i = 0; i < n; ++i) {
            d[order[i]] = i;
        }
        return Arrays.stream(friends)
            .boxed()
            .sorted((a, b) -> d[a] - d[b])
            .mapToInt(Integer::intValue)
            .toArray();
    }
}

// Accepted solution for LeetCode #3668: Restore Finishing Order
func recoverOrder(order []int, friends []int) []int {
	n := len(order)
	d := make([]int, n+1)
	for i, x := range order {
		d[x] = i
	}
	sort.Slice(friends, func(i, j int) bool {
		return d[friends[i]] < d[friends[j]]
	})
	return friends
}

# Accepted solution for LeetCode #3668: Restore Finishing Order
class Solution:
    def recoverOrder(self, order: List[int], friends: List[int]) -> List[int]:
        d = {x: i for i, x in enumerate(order)}
        return sorted(friends, key=lambda x: d[x])

// Accepted solution for LeetCode #3668: Restore Finishing Order
fn recover_order(order: Vec<i32>, friends: Vec<i32>) -> Vec<i32> {
    use std::collections::HashSet;

    let s: HashSet<_> = friends.into_iter().collect();
    order.into_iter().filter(|n| s.contains(n)).collect()
}

fn main() {
    let order = vec![3, 1, 2, 5, 4];
    let friends = vec![1, 3, 4];
    let ret = recover_order(order, friends);
    println!("ret={ret:?}");
}

#[test]
fn test() {
    {
        let order = vec![3, 1, 2, 5, 4];
        let friends = vec![1, 3, 4];
        let ret = recover_order(order, friends);
        assert_eq!(ret, vec![3, 1, 4]);
    }
    {
        let order = vec![1, 4, 5, 3, 2];
        let friends = vec![2, 5];
        let ret = recover_order(order, friends);
        assert_eq!(ret, vec![5, 2]);
    }
}

// Accepted solution for LeetCode #3668: Restore Finishing Order
function recoverOrder(order: number[], friends: number[]): number[] {
    const n = order.length;
    const d: number[] = Array(n + 1).fill(0);
    for (let i = 0; i < n; ++i) {
        d[order[i]] = i;
    }
    return friends.sort((a, b) => d[a] - d[b]);
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n × log n)

Space

O(n)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED

O(n) time

O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.

Mutating counts without cleanup

Wrong move: Zero-count keys stay in map and break distinct/count constraints.

Usually fails on: Window/map size checks are consistently off by one.

Fix: Delete keys when count reaches zero.