leetcode 91. Decode Ways

最新推荐文章于 2020-02-27 10:19:39 发布

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本文介绍了一种用于解码数字字符串的算法，该算法能够计算出给定数字串的所有可能解码方式的数量。例如，对于数字串“12”，它可以被解码为“AB”或者“L”，因此解码方式共有两种。

Question

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.

Code

public int numDecodings(String s) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        int[] nums = new int[s.length() + 1];
        nums[0] = 1;
        nums[1] = s.charAt(0) != '0' ? 1 : 0;
        for (int i = 2; i <= s.length(); i++) {
            if (s.charAt(i - 1) != '0') {
                nums[i] += nums[i - 1];
            }
            int twoDigits = (s.charAt(i - 2) - '0') * 10 + s.charAt(i - 1) - '0';
            if (twoDigits >= 10 && twoDigits <= 26) {
                nums[i] += nums[i - 2];
            }
        }
        return nums[s.length()];
    }