add solution for valid-palindrome

Author: Ujoonnee

valid-palindrome/Ujoonnee.java

@@ -0,0 +1,51 @@
+// time complexity : O(n)
+// space complexity : O(1)
+
+class Solution {
+    public boolean isPalindrome(String s) {
+        s = s.toLowerCase();
+        int i = 0;
+        int j = s.length() - 1;
+        while(i < j) {
+            char c1 = s.charAt(i);
+            if (!isAlphanumeric(c1)) {
+                i++;
+                continue;
+            }
+
+            char c2 = s.charAt(j);
+            if (!isAlphanumeric(c2)) {
+                j--;
+                continue;
+            }
+
+            if (c1 != c2) {
+                return false;
+            }
+            i++;
+            j--;
+        }
+
+        return true;
+    }
+
+    private boolean isAlphanumeric(char c) {
+        return (c >= '0' && c<='9') || (c >= 'A' && c <= 'Z') || (c >= 'a' && c<= 'z');
+    }
+}
+
+/*
+class Solution {
+    public boolean isPalindrome(String s) {
+        s = s.toLowerCase();
+        String[] split = s.replaceAll("[^A-Za-z0-9]", "").split("");
+        for (int i=0; i<split.length/2; i++) {
+            if (!split[i].equals(split[split.length-1-i])) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}
+*/