g0201_0300.s0210_course_schedule_ii.Solution.kt Maven / Gradle / Ivy

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Kotlin-based LeetCode algorithm problem solutions, regularly updated

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package g0201_0300.s0210_course_schedule_ii

// #Medium #Top_Interview_Questions #Depth_First_Search #Breadth_First_Search #Graph
// #Topological_Sort #Level_2_Day_11_Graph/BFS/DFS
// #2022_10_20_Time_266_ms_(96.32%)_Space_45.9_MB_(92.65%)

class Solution {
    fun findOrder(numCourses: Int, prerequisites: Array): IntArray {
        val indegrees = IntArray(numCourses) { 0 }
        val graph = buildGraph(numCourses, prerequisites, indegrees)
        val queue = ArrayDeque()
        for ((idx, indegree) in indegrees.withIndex()) {
            if (indegree == 0) {
                queue.addLast(idx)
            }
        }
        val ans = IntArray(numCourses) { 0 }
        var idx = 0
        while (queue.isNotEmpty()) {
            val cur = queue.removeFirst()
            ans[idx++] = cur
            for (pre in graph[cur]) {
                if (--indegrees[pre] == 0) {
                    queue.addLast(pre)
                }
            }
        }
        if (idx < numCourses) {
            return intArrayOf()
        }
        return ans
    }

    private fun buildGraph(
        numCourses: Int,
        prerequisites: Array,
        indegrees: IntArray
    ): List> {
        val graph = List(numCourses) { mutableListOf() }
        for ((cur, prev) in prerequisites) {
            graph[prev].add(cur)
            ++indegrees[cur]
        }
        return graph
    }
}