[eunhwa99] Week 12 solutions

non-overlapping-intervals/eunhwa99.kt

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+class Solution {
+    // TC: O(n log n) - intervals를 정렬하는데 O(n log n)
+    // SC: O(1) - 추가적인 공간을 사용하지 않음
+    fun eraseOverlapIntervals(intervals: Array<IntArray>): Int {
+        intervals.sortBy { it[1] } // intervals를 끝나는 시간 기준으로 정렬
+
+        var prevEnd = intervals[0][1]
+        var count = 0
+        for (i in 1 until intervals.size) {
+            val curStart = intervals[i][0]
+            if (curStart < prevEnd) { // 두 구간이 겹친다.
+                count++
+            } else {
+                prevEnd = intervals[i][1] // 겹치지 않으면 prevEnd를 갱신
+            }
+
+        }
+        return count
+    }
+}

number-of-connected-components-in-an-undirected-graph/eunhwa99.kt

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+class Solution {
+    // Time Complexity: O(n + e)
+    // Space Complexity: O(n + e)
+    // n : number of nodes
+    // e : number of edges
+    fun countComponents(n: Int, edges: Array<IntArray>): Int {
+        val graph = Array(n) { mutableListOf<Int>() }
+        for (edge in edges) { // make graph
+            graph[edge[0]].add(edge[1])
+            graph[edge[1]].add(edge[0])
+        }
+        val visited = BooleanArray(n) // visited array
+        var count = 0 // number of connected components
+
+        for (i in 0 until n) {
+            if (!visited[i]) {
+                dfs(i, graph, visited)
+                count++
+            }
+        }
+        return count
+    }
+
+    fun dfs(node: Int, graph: Array<MutableList<Int>>, visited: BooleanArray) {
+        visited[node] = true
+        for (neighbor in graph[node]) {
+            if (!visited[neighbor]) {
+                dfs(neighbor, graph, visited)
+            }
+        }
+    }
+}

remove-nth-node-from-end-of-list/eunhwa99.kt

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+package leetcode_study
+
+class ListNode(var `val`: Int) {
+    var next: ListNode? = null
+}
+
+// 힌트 참고
+// Time Complexity: O(L) where L is the length of the linked list
+// Space Complexity: O(1) since we are using only a constant amount of space
+class Solution {
+    fun removeNthFromEnd(head: ListNode?, n: Int): ListNode? {
+        var left = head
+        var right = head
+        var step = n
+        while (step-- > 0) {
+            right = right?.next
+        }
+        if (right == null) {
+            return head?.next // n이 리스트의 길이와 같을 때, 즉 첫 번째 노드를 제거해야 하는 경우
+        }
+        while (right?.next != null) {
+            left = left?.next
+            right = right.next
+        }
+
+        left?.next = left?.next?.next
+        return head
+    }
+}
+
+

same-tree/eunhwa99.kt

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+package leetcode_study
+
+class TreeNode(var `val`: Int) {
+    var left: TreeNode? = null
+    var right: TreeNode? = null
+}
+
+// Time Complexity: O(n)
+// Space Complexity: O(h) where h is the height of the tree (재귀함수로 인한 공간 복잡도)
+class Solution {
+    fun isSameTree(p: TreeNode?, q: TreeNode?): Boolean {
+        return if (p == null && q == null) {
+            true
+        } else if (p == null || q == null || p.`val` != q.`val`) {
+            false
+        } else {
+            isSameTree(p.left, q.left) && isSameTree(p.right, q.right)
+        }
+    }
+}

serialize-and-deserialize-binary-tree/eunhwa99.kt

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+package leetcode_study
+
+class Codec {
+    private var index = 0
+    // Time Complexity: O(n)
+    // Space Complexity: O(n)
+    fun serialize(root: TreeNode?): String {
+        return buildString {
+            traverse(root, this)
+        }
+    }
+
+    fun traverse(node: TreeNode?, sb: StringBuilder) {
+        if (node == null) {
+            sb.append("null,")
+            return
+        }
+        sb.append("${node.`val`},")
+        traverse(node.left, sb)
+        traverse(node.right, sb)
+    }
+
+    // Time Complexity: O(n)
+    // Space Complexity: O(n)
+    fun deserialize(data: String): TreeNode? {
+        val values = data.split(",").filter { it.isNotEmpty() }.toList()
+        index = 0
+        return buildTree(values)
+    }
+
+    fun buildTree(values: List<String>): TreeNode? {
+        if (index >= values.size) return null
+        val value = values[index++] // index 는 매개변수가 아니라 전역으로 두어야 한다!!
+        if (value == "null") return null
+        val node = TreeNode(value.toInt())
+        node.left = buildTree(values)
+        node.right = buildTree(values)
+        return node
+    }
+}
+