LeetCode, leetcodeoj

LeetCode, leetcodeoj

Question:

A message containing letters fromA-ZIs 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 cocould be decoded"AB"(1 2) or"L"(12 ).

The number of ways decoding"12"Is 2.

Ideas:

Analysis

1. The current character is 0

The first character of 1.1 is 0, 3-9.

The first character of 1.2 is 1, 2.

2. The current character is not 0

The first character of 2.1 is 0, 3-9.

The first character of 2.2 is 2, and the current character is 7-9

2.3 remaining

package dp;public class DecodeWays {    public int numDecodings(String s) {        int n;        if (s == null || (n = s.length()) == 0) return 0;        int[] dp = new int[n + 1];        dp[0] = 1;        if (s.charAt(0) == '0') return 0;        dp[1] = 1;        for (int i = 2; i <= n; ++i) {            if (s.charAt(i - 1) == '0') {                if (s.charAt(i - 2) != '1' && s.charAt(i - 2) != '2') return 0;                dp[i] = dp[i - 2];            } else if (s.charAt(i - 2) == '0' || s.charAt(i - 2) > '2' ||                     (s.charAt(i - 2) == '2' && s.charAt(i - 1) > '6')) {                dp[i] = dp[i - 1];            } else {                dp[i] = dp[i - 1] + dp[i - 2];            }        }        return dp[n];    }        public static void main(String[] args) {        // TODO Auto-generated method stub        DecodeWays d = new DecodeWays();        System.out.println(d.numDecodings("10") == 1);        System.out.println(d.numDecodings("101") == 1);        System.out.println(d.numDecodings("1001") == 0);        System.out.println(d.numDecodings("10012") == 0);        System.out.println(d.numDecodings("0012") == 0);        System.out.println(d.numDecodings("12") == 2);        System.out.println(d.numDecodings("128") == 2);        System.out.println(d.numDecodings("27") == 1);        System.out.println(d.numDecodings("99") == 1);    }}

Simpler:

package dp;public class DecodeWays {    public int numDecodings(String s) {        int n;        if (s == null || (n = s.length()) == 0) return 0;        int[] dp = new int[n + 1];        dp[0] = 1;        if (s.charAt(0) == '0') return 0;        dp[1] = 1;        for (int i = 2; i <= n; ++i) {            int c1 = 0;            if (s.charAt(i - 1) != '0')                c1 = dp[i - 1];            int c2 = 0;            if (s.charAt(i - 2) == '1' || (s.charAt(i - 2) == '2' && s.charAt(i - 1) < '7'))                c2 = dp[i - 2];            dp[i] = c1 + c2;        }                return dp[n];    }        public static void main(String[] args) {        // TODO Auto-generated method stub        DecodeWays d = new DecodeWays();        System.out.println(d.numDecodings("10") == 1);        System.out.println(d.numDecodings("101") == 1);        System.out.println(d.numDecodings("1001") == 0);        System.out.println(d.numDecodings("10012") == 0);        System.out.println(d.numDecodings("0012") == 0);        System.out.println(d.numDecodings("12") == 2);        System.out.println(d.numDecodings("128") == 2);        System.out.println(d.numDecodings("27") == 1);        System.out.println(d.numDecodings("99") == 1);    }}