Leetcode-91. Decode Ways

前言：为了后续的实习面试，开始疯狂刷题，非常欢迎志同道合的朋友一起交流。因为时间比较紧张，目前的规划是先过一遍，写出能想到的最优算法，第二遍再考虑最优或者较优的方法。如有错误欢迎指正。博主首发CSDN，mcf171专栏。

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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.

这个题目直接考虑从左往右解。通过维护一个Map来进行维护避免重复操作。Your runtime beats 28.37% of java submissions.

public class Solution {    public int numDecodings(String s) {        if(s.length()==0) return 0;        if(s.length() == 1) if(!(s.equals("0"))) return 1; else return 0;        if(s.charAt(0) == '0') return 0;        Map<Integer,Integer> i2results = new HashMap<Integer,Integer>();        return generateSubString(s,0,s.length() - 1,i2results);    }    public int generateSubString(String s, int i,int end,Map<Integer,Integer> i2results){        if (i2results.containsKey(i)) return i2results.get(i);        if(i == end )if(s.charAt(i) != '0') return 1; else  return 0;        if(i == end - 1){            if(s.charAt(i) - '0' > 2) return generateSubString(s,i+1,end,i2results);            if(s.charAt(i) == '2'){                if(s.charAt(i+1) - '0' >= 7) return 1;                if(s.charAt(i+1) == '0') return 1;                return 2;            }            if(s.charAt(i) == '1'){                if(s.charAt(i+1) - '0' != 0) return 2;                if(s.charAt(i+1) == '0') return 1;            }            if (s.charAt(i) == '0') return 0;        }else{            int results = 0;            if(s.charAt(i) - '0' > 2) results += generateSubString(s,i+1,end,i2results);            if(s.charAt(i) == '2'){                if(s.charAt(i+1) - '0' >= 7) results += generateSubString(s,i+1,end,i2results);                else if(s.charAt(i+1) == '0') results += generateSubString(s,i+2,end,i2results);                else {                    results += generateSubString(s,i+1,end,i2results);                    results += generateSubString(s,i+2,end,i2results);                }            }            if(s.charAt(i) == '1'){                if(s.charAt(i+1) - '0' != 0) {                    results += generateSubString(s,i+1,end,i2results);                    results += generateSubString(s,i+2,end,i2results);                }                if(s.charAt(i+1) == '0') results += generateSubString(s,i+2,end,i2results);            }            if (s.charAt(i) == '0') return 0;            i2results.put(i,results);            return results;        }        return 0;    }}