DecodeWays

Change-Id: I48bcd05ae17fbc67983c3186c7de3f7045e0204c

Author: applewjg

DecodeWays.java

@@ -0,0 +1,38 @@
+/*
+ Author:     King, [email protected]
+ Date:       Dec 25, 2014
+ Problem:    Decode Ways
+ Difficulty: Easy
+ Source:     https://oj.leetcode.com/problems/decode-ways/
+ Notes:
+ A message containing letters from A-Z is being encoded to numbers using the following mapping:
+ 'A' -> 1
+ 'B' -> 2
+ ...
+ 'Z' -> 26
+ Given an encoded message containing digits, determine the total number of ways to decode it.
+ For example,
+ Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).
+ The number of ways decoding "12" is 2.
+
+ Solution: 1. dp. Time : O(n); Space : O(1).
+ */
+
+public class Solution {
+    public int numDecodings(String s) {
+        if (s.length() == 0 || s.charAt(0) == '0') return 0;
+        int N = s.length();
+        int f0 = 1, f1 = 1;
+        for (int i = 1; i < N; ++i) {
+            if (s.charAt(i) == '0') f1 = 0;
+            int num = s.charAt(i) - '0' + (s.charAt(i-1) - '0') * 10;
+            if (num < 10 || num > 26) {
+                f0 = 0;
+            }
+            int tmp = f1;
+            f1 = f1 + f0;
+            f0 = tmp;
+        }
+        return f1;
+    }
+}