Longest common subpath

Array Binary Search Rolling Hash Suffix Array Hash Function

29.4% acceptance

Feb 25, 2026

511

There is a country of n cities numbered from 0 to n - 1. In this country, there is a road connecting every pair of cities. There are m friends numbered from 0 to m - 1 who are traveling through the country. Each one of them will take a path consisting of some cities. Each path is represented by an integer array that contains the visited cities in order. The path may contain a city more than once, but the same city will not be listed consecutively. Given an integer n and a 2D integer array paths where paths[i] is an integer array representing the path of the ith friend, return the length of the longest common subpath that is shared by every friend's path, or 0 if there is no common subpath at all. A subpath of a path is a contiguous sequence of cities within that path.

Solution

Rust

Time O(n²)

Space O(n)

LeetCode

solution.rs

use std::collections::HashSet;

impl Solution {
  pub fn longest_common_subpath(_n: i32, paths: Vec<Vec<i32>>) -> i32 {
    let min_len = paths.iter().map(|p| p.len()).min().unwrap();
    if min_len == 0 {
      return 0;
    }

    let (mut lo, mut hi) = (0usize, min_len);
    while lo < hi {
      let mid = lo + (hi - lo + 1) / 2;
      if Self::check(&paths, mid) {
        lo = mid;
      } else {
        hi = mid - 1;
      }
    }
    lo as i32
  }

  fn check(paths: &[Vec<i32>], len: usize) -> bool {
    if len == 0 {
      return true;
    }
    // Use double hashing to avoid collisions
    const MOD1: u64 = 1_000_000_007;
    const MOD2: u64 = 998_244_353;
    const BASE1: u64 = 100_003;
    const BASE2: u64 = 100_019;

    let mut common: Option<HashSet<(u64, u64)>> = None;

    for path in paths {
      if path.len() < len {
        return false;
      }
      let mut set = HashSet::new();
      let mut h1 = 0u64;
      let mut h2 = 0u64;
      let mut pow1 = 1u64;
      let mut pow2 = 1u64;

      for i in 0..len {
        h1 = (h1 * BASE1 + path[i] as u64 + 1) % MOD1;
        h2 = (h2 * BASE2 + path[i] as u64 + 1) % MOD2;
        if i > 0 {
          pow1 = pow1 * BASE1 % MOD1;
          pow2 = pow2 * BASE2 % MOD2;
        }
      }
      set.insert((h1, h2));

      for i in len..path.len() {
        h1 = (h1 + MOD1 - pow1 * (path[i - len] as u64 + 1) % MOD1) % MOD1;
        h1 = (h1 * BASE1 + path[i] as u64 + 1) % MOD1;
        h2 = (h2 + MOD2 - pow2 * (path[i - len] as u64 + 1) % MOD2) % MOD2;
        h2 = (h2 * BASE2 + path[i] as u64 + 1) % MOD2;
        set.insert((h1, h2));
      }

      common = Some(match common {
        None => set,
        Some(prev) => prev.intersection(&set).copied().collect(),
      });
      if common.as_ref().unwrap().is_empty() {
        return false;
      }
    }
    !common.unwrap().is_empty()
  }
}