[hyunjung-choi] WEEK 05 solutions

best-time-to-buy-and-sell-stock/hyunjung-choi.kt

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+/**
+ * 시간 복잡도: O(n)
+ * 공간 복잡도: O(1)
+ */
+
+class Solution {
+    fun maxProfit(prices: IntArray): Int {
+        var minPrice = Int.MAX_VALUE
+        var maxProfit = 0
+
+        prices.forEach { price ->
+            minPrice = minOf(minPrice, price)
+            maxProfit = maxOf(maxProfit, price - minPrice)
+        }
+
+        return maxProfit
+    }
+}

group-anagrams/hyunjung-choi.kt

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+/**
+ * N: 단어 개수, M: 평균 단어 길이
+ * 시간 복잡도: O(N × M log M)
+ * 공간 복잡도: O(N × M)
+ */
+
+class Solution {
+    fun groupAnagrams(strs: Array<String>): List<List<String>> {
+        val groupMap = mutableMapOf<String, MutableList<String>>()
+
+        for (str in strs) {
+            val sortedKey = str.toCharArray().sorted().joinToString("")
+            groupMap.getOrPut(sortedKey) { mutableListOf()}.add(str)
+        }
+
+        return groupMap.values.toList()
+    }
+}

implement-trie-prefix-tree/hyunjung-choi.kt

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+/**
+ * 시간복잡도: O(L) (insert, search, startsWith 모두 동일)
+ * 공간복잡도: O(ΣL × alphabetSize)
+ */
+
+class Trie() {
+
+    private class Node {
+        val child = arrayOfNulls<Node>(26)
+        var isEnd: Boolean = false
+    }
+
+    private val root = Node()
+
+    fun insert(word: String) {
+        var cur = root
+        for (ch in word) {
+            val i = ch - 'a'
+            if (cur.child[i] == null) {
+                cur.child[i] = Node()
+            }
+            cur = cur.child[i]!!
+        }
+        cur.isEnd = true
+    }
+
+    fun search(word: String): Boolean {
+        val node = findNode(word)
+        return node?.isEnd == true
+    }
+
+    fun startsWith(prefix: String): Boolean {
+        return findNode(prefix) != null
+    }
+
+    private fun findNode(s: String): Node? {
+        var cur = root
+        for (ch in s) {
+            val i = ch - 'a'
+            val next = cur.child[i] ?: return null
+            cur = next
+        }
+        return cur
+    }
+}