Merge pull request #1805 from std-freejia/main

[std-freejia] week 03 solutions

Author: std-freejia

decode-ways/std-freejia.java

@@ -0,0 +1,26 @@
+class Solution {
+    public int numDecodings(String s) {
+        // 0 으로 시작하거나 빈 문자열이면 해독불가
+        int sLen =  s.length();
+        if (sLen == 0 || s.charAt(0) == '0') return 0;
+
+        int[] dp =  new int[sLen+1];
+
+        dp[0] = 1;
+        dp[1] = 1;
+
+        for (int i = 2; i <= sLen; i++) {
+
+            int num = Integer.parseInt(s.substring(i-2, i));
+            // 1자리 숫자로 해독 가능한 경우
+            if (s.charAt(i-1) != '0') {
+                dp[i] += dp[i-1];
+            }
+            // 2자리 숫자로 해독 가능한 경우
+            if (num >= 10 && num <= 26) {
+                dp[i] += dp[i-2];
+            }
+        }
+        return dp[sLen];
+    }
+}

number-of-1-bits/std-freejia.java

@@ -0,0 +1,10 @@
+class Solution {
+    public int hammingWeight(int n) {
+        int count = 0;
+        while (n > 0) {
+            if ((n & 1) == 1) count++;
+            n >>= 1;
+        }
+        return count;
+    }
+}

valid-palindrome/std-freejia.java

@@ -0,0 +1,6 @@
+class Solution {
+    public boolean isPalindrome(String s) {
+        String alphabetOnly = s.replaceAll("[^a-zA-Z0-9]", "").toLowerCase();
+        return new StringBuilder(alphabetOnly).reverse().toString().equals(alphabetOnly);
+    }
+}