modify code

src/class19/Code02_ConvertToLetterString.java

@@ -30,7 +30,10 @@ public class Code02_ConvertToLetterString {
 		return ways;
 	}
 
-	public static int dp(String s) {
+	// 从右往左的动态规划
+	// 就是上面方法的动态规划版本
+	// dp[i]表示：str[i...]有多少种转化方式
+	public static int dp1(String s) {
 		if (s == null || s.length() == 0) {
 			return 0;
 		}
@@ -50,9 +53,66 @@ public class Code02_ConvertToLetterString {
 		return dp[0];
 	}
 
+	// 从左往右的动态规划
+	// dp[i]表示：str[0...i]有多少种转化方式
+	public static int dp2(String s) {
+		if (s == null || s.length() == 0) {
+			return 0;
+		}
+		char[] str = s.toCharArray();
+		int N = str.length;
+		if (str[0] == '0') {
+			return 0;
+		}
+		int[] dp = new int[N];
+		dp[0] = 1;
+		for (int i = 1; i < N; i++) {
+			if (str[i] == '0') {
+				if (str[i - 1] == '0' || str[i - 1] > '2' || (i - 2 >= 0 && dp[i - 2] == 0)) {
+					return 0;
+				} else {
+					dp[i] = i - 2 >= 0 ? dp[i - 2] : 1;
+				}
+			} else {
+				dp[i] = dp[i - 1];
+				if (str[i - 1] != '0' && (str[i - 1] - '0') * 10 + str[i] - '0' <= 26) {
+					dp[i] += i - 2 >= 0 ? dp[i - 2] : 1;
+				}
+			}
+		}
+		return dp[N - 1];
+	}
+
+	// 为了测试
+	public static String randomString(int len) {
+		char[] str = new char[len];
+		for (int i = 0; i < len; i++) {
+			str[i] = (char) ((int) (Math.random() * 10) + '0');
+		}
+		return String.valueOf(str);
+	}
+
+	// 为了测试
 	public static void main(String[] args) {
-		System.out.println(number("7210231231232031203123"));
-		System.out.println(dp("7210231231232031203123"));
+		int N = 30;
+		int testTime = 1000000;
+		System.out.println("测试开始");
+		for (int i = 0; i < testTime; i++) {
+			int len = (int) (Math.random() * N);
+			String s = randomString(len);
+			int ans0 = number(s);
+			int ans1 = dp1(s);
+			int ans2 = dp2(s);
+			if (ans0 != ans1 || ans0 != ans2) {
+				System.out.println(s);
+				System.out.println(ans0);
+				System.out.println(ans1);
+				System.out.println(ans2);
+				System.out.println("Oops!");
+				break;
+			}
+		}
+		System.out.println("测试结束");
 	}
 
 }