91. Decode Ways

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.

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Solution:

Tips:

f(n) = f(n - 1) + f(n - 2) for character our of range 'A' to 'Z' needs to specific dealing.

Java Code:

public class Solution {
public int numDecodings(String s) {
if (s == null || s.length() < 1 || s.charAt(0) == '0') {
return 0;
}

int p = 1;
int q = 1;
int result = 1;
for (int i = 1; i < s.length(); i++) {
char cc = s.charAt(i);
char pc = s.charAt(i - 1);

if ((pc > '2' || pc == '0') && cc == '0') {
return 0;
}

if (cc == '0') {
result = p;
} else if (pc == '1' || pc == '2' && cc <= '6') {
result = p + q;
} else {
// pc == '0' || pc == '2' && cc == 7 8 9 || pc >= 3
result = q;
}
p = q;
q = result;
}

return result;
}
}