CODE 43. Decode Ways

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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.

public int numDecodings(String s) {
	// Start typing your Java solution below
	// DO NOT write main() function
	if (null == s || "".equals(s)) {
		return 0;
	}
	int[] num = new int[s.length() + 1];
	char[] cs = s.toCharArray();
	num[0] = 1;
	if (cs[0] > '0') {
		num[1] = 1;
	} else {
		num[1] = 0;
	}
	for (int i = 2; i < s.length() + 1; i++) {
		int num1 = 0;
		int num2 = 0;
		if (cs[i - 1] != '0') {
			num1 = num[i - 1];
		}
		if (cs[i - 2] == '1') {
			num2 = num[i - 2];
		} else if (cs[i - 2] == '2'
				&& (cs[i - 1] >= '0' && cs[i - 1] <= '6')) {
			num2 = num[i - 2];
		}
		num[i] = num1 + num2;
	}
	return num[s.length()];
}