91.decode-ways.java

/*
 * @lc app=leetcode id=91 lang=java
 *
 * [91] Decode Ways
 *
 * https://leetcode.com/problems/decode-ways/description/
 *
 * algorithms
 * Medium (21.89%)
 * Total Accepted:    249.7K
 * Total Submissions: 1.1M
 * Testcase Example:  '"12"'
 *
 * A message containing letters from A-Z is being encoded to numbers using the
 * following mapping:
 *
 *
 * 'A' -> 1
 * 'B' -> 2
 * ...
 * 'Z' -> 26
 *
 *
 * Given a non-empty string containing only digits, determine the total number
 * of ways to decode it.
 *
 * Example 1:
 *
 *
 * Input: "12"
 * Output: 2
 * Explanation: It could be decoded as "AB" (1 2) or "L" (12).
 *
 *
 * Example 2:
 *
 *
 * Input: "226"
 * Output: 3
 * Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2
 * 6).
 *
 */
class Solution {
  public int numDecodings(String s) {
    if (s == null || s.length() == 0 || s.charAt(0) == '0') {
      return 0;
    }

    int n = s.length();
    int[] dp = new int[n + 1];
    dp[n] = 1;
    dp[n - 1] = (s.charAt(n - 1) == '0') ? 0 : 1;

    for (int i = n - 2; i >= 0; i--) {
      if (s.charAt(i) != '0') {
        int temp = Integer.valueOf(s.substring(i, i + 2));
        if (temp > 26) {
          dp[i] = dp[i + 1];
        } else {
          dp[i] = dp[i + 1] + dp[i + 2];
        }
      }
    }

    return dp[0];
  }
}