[Leetcode] Decode ways

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.

Https://oj.leetcode.com/problems/decode-ways/

 

Idea 1: DFS.

Idea 2: DP. Like the Fibonacci series, DP [I] indicates the total number of combinations from the beginning to the I bit.

If the I-1 bit is not equal to 0, DP [I] needs to add DP [I-1.

If the I-2 bit to the I-1 bit is between 1-26, DP [I] needs to add the DP [I-2] situation.

 

public class Solution {    // be careful of "0" "01" "100"    public int numDecodings(String s) {        if (s.length() == 0)            return 0;        int[] dp = new int[s.length() + 1];        dp[0] = 1;        if (s.charAt(0) != ‘0‘)            dp[1] = 1;        else            dp[1] = 0;        for (int i = 1; i < s.length(); i++) {            if (s.charAt(i) != ‘0‘)                dp[i + 1] = dp[i];            if (s.charAt(i - 1) != ‘0‘ && Integer.parseInt(s.substring(i - 1, i + 1)) <= 26)                dp[i + 1] += dp[i - 1];        }        return dp[s.length()];    }    public static void main(String[] args) {        String s = "1010";        System.out.println(new Solution().numDecodings(s));    }}

View code

 

Refer:

Http://blog.csdn.net/u011095253/article/details/9248109

Http://blog.csdn.net/worldwindjp/article/details/19938131