solve week 1 problems

- 0001. Two Sum
- 0121. Best Time to Buy and Sell Stock
- 0125. Valid Palindrome
- 0217. Contains Duplicate
- 0242. Valid Anagram

Author: borahm-lee

best-time-to-buy-and-sell-stock/bky373.java

@@ -0,0 +1,22 @@
+/**
+ * https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
+ * TC : O(N)
+ * SC : O(1)
+ */
+class Solution_0121 {
+    public int maxProfit(int[] prices) {
+        int hold = prices[0];
+        int profit = 0;
+
+        for (int i = 1; i < prices.length; i++) {
+            if (prices[i] < hold) {
+                hold = prices[i];
+            }
+
+            if (prices[i] - hold > profit) {
+                profit = prices[i] - hold;
+            }
+        }
+        return profit;
+    }
+}

contains-duplicate/bky373.java

@@ -0,0 +1,22 @@
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * https://leetcode.com/problems/contains-duplicate/
+ * TC : O(N)
+ * SC : O(N)
+ */
+class Solution_0217 {
+
+    public boolean containsDuplicate(int[] nums) {
+        Map<Integer, Integer> map = new HashMap();
+        for (int n : nums) {
+            if (map.containsKey(n)) {
+                return true;
+            } else {
+                map.put(n, 1);
+            }
+        }
+        return false;
+    }
+}

two-sum/bky373.java

@@ -0,0 +1,22 @@
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * https://leetcode.com/problems/two-sum/
+ * TC : O(N)
+ * SC : O(N)
+ */
+class Solution_0001 {
+
+    public int[] twoSum(int[] nums, int target) {
+        Map<Integer, Integer> map = new HashMap<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            if (map.containsKey(target - nums[i])) {
+                return new int[]{map.get(target - nums[i]), i};
+            }
+            map.put(nums[i], i);
+        }
+        return new int[]{};
+    }
+}

valid-anagram/bky373.java

@@ -0,0 +1,26 @@
+/**
+ * https://leetcode.com/problems/valid-anagram/
+ * TC : O(N)
+ * SC : O(1)
+ */
+class Solution_242 {
+    public boolean isAnagram(String s, String t) {
+        if (s.length() != t.length()) {
+            return false;
+        }
+
+        int[] alpCnt = new int[26];
+
+        for (int i=0; i<s.length(); i++) {
+            alpCnt[s.charAt(i) - 'a']++;
+        }
+
+        for (int i = 0; i < s.length(); i++) {
+            if (alpCnt[t.charAt(i) - 'a'] < 1) {
+                return false;
+            }
+            alpCnt[t.charAt(i) - 'a']--;
+        }
+        return true;
+    }
+}

valid-palindrome/bky373.java

@@ -0,0 +1,36 @@
+/**
+ * https://leetcode.com/problems/valid-palindrome/
+ * TC : O(N)
+ * SC : O(1)
+ */
+class Solution_0125 {
+
+    public boolean isPalindrome(String s) {
+        if (s.isBlank() || s.length() == 1) {
+            return true;
+        }
+        s = s.toLowerCase();
+
+        int j = s.length() - 1;
+        for (int i = 0; i < j; i++) {
+            if (!isAlpNum(s.charAt(i))) {
+                continue;
+            }
+            if (!isAlpNum(s.charAt(j))) {
+                i--;
+                j--;
+                continue;
+            }
+            if (s.charAt(i) != s.charAt(j)) {
+                return false;
+            }
+            j--;
+        }
+
+        return true;
+    }
+
+    public boolean isAlpNum(char c) {
+        return ('0' <= c && c <= '9') || ('a' <= c && c <= 'z');
+    }
+}