LeetCode in Kotlin

1923. Longest Common Subpath

Hard

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.

Example 1:

Input: n = 5, paths = [[0,1,2,3,4], [2,3,4], [4,0,1,2,3]]

Output: 2

Explanation: The longest common subpath is [2,3].

Example 2:

Input: n = 3, paths = [[0],[1],[2]]

Output: 0

Explanation: There is no common subpath shared by the three paths.

Example 3:

Input: n = 5, paths = [[0,1,2,3,4], [4,3,2,1,0]]

Output: 1

Explanation: The possible longest common subpaths are [0], [1], [2], [3], and [4]. All have a length of 1.

Constraints:

• 1 <= n <= 105
• m == paths.length
• 2 <= m <= 105
• sum(paths[i].length) <= 105
• 0 <= paths[i][j] < n
• The same city is not listed multiple times consecutively in paths[i].

Solution

@Suppress("UNUSED_PARAMETER")
class Solution {
    private lateinit var pow: LongArray

    fun longestCommonSubpath(n: Int, paths: Array<IntArray>): Int {
        var res = 0
        var min = Int.MAX_VALUE
        for (path in paths) {
            min = Math.min(min, path.size)
        }
        pow = LongArray(min + 1)
        pow[0]++
        for (i in 1..min) {
            pow[i] = pow[i - 1] * BASE % MOD
        }
        var st = 1
        var end = min
        var mid = (st + end) / 2
        while (st <= end) {
            if (commonSubstring(paths, mid)) {
                res = mid
                st = mid + 1
            } else {
                end = mid - 1
            }
            mid = (st + end) / 2
        }
        return res
    }

    private fun commonSubstring(paths: Array<IntArray>, l: Int): Boolean {
        val set = rollingHash(paths[0], l)
        var i = 1
        val n = paths.size
        while (i < n) {
            set.retainAll(rollingHash(paths[i], l))
            if (set.isEmpty()) {
                return false
            }
            i++
        }
        return true
    }

    private fun rollingHash(a: IntArray, l: Int): HashSet<Long> {
        val set = HashSet<Long>()
        var hash: Long = 0
        for (i in 0 until l) {
            hash = (hash * BASE + a[i]) % MOD
        }
        set.add(hash)
        val n = a.size
        var curr = l
        var prev = 0
        while (curr < n) {
            hash = (hash * BASE % MOD - a[prev] * pow[l] % MOD + a[curr] + MOD) % MOD
            set.add(hash)
            prev++
            curr++
        }
        return set
    }

    companion object {
        private const val BASE: Long = 100001
        private val MOD = (Math.pow(10.0, 11.0) + 7).toLong()
    }
}

This site is open source. Improve this page.