[Yiseull] WEEK 03 Solutions

number-of-1-bits/Yiseull.java

@@ -0,0 +1,5 @@
+class Solution {
+    public int hammingWeight(int n) {
+        return Integer.bitCount(n);
+    }
+}

valid-palindrome/Yiseull.java

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+class Solution {
+    public boolean isPalindrome(String s) {
+        /**
+        첫 번째 풀이: 시간복잡도 O(n), 공간복잡도 O(n)
+        String letters = s.replaceAll("[^a-zA-Z0-9]", "").toLowerCase();
+        String reverse = new StringBuilder(letters).reverse().toString();
+        return letters.equals(reverse);
+         */
+
+        // 두 번째 풀이: 시간복잡도 O(n), 공간복잡도 O(1)
+         int left = 0, right = s.length() - 1;
+         while (left < right) {
+            while (left < right && !Character.isLetterOrDigit(s.charAt(left))) {
+                left++;
+            }
+
+            while (left < right && !Character.isLetterOrDigit(s.charAt(right))) {
+                right--;
+            }
+
+            if (Character.toLowerCase(s.charAt(left)) != Character.toLowerCase(s.charAt(right))) {
+                return false;
+            }
+
+            left++;
+            right--;
+         }
+
+         return true;
+    }
+}