leetcode/91.decode-ways.java at main · jameszhan02/leetcode · GitHub

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/*
 * @lc app=leetcode id=91 lang=java
 *
 * [91] Decode Ways
 */

// @lc code=start
class Solution {
    // 2 1 0 1
    // 1 2 1 1
    // 1 0
    // 1 1
    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[0] = 1;
        dp[1] = 1;

        for (int i = 2; i <= n; ++i) {
            int oneDigit = s.charAt(i - 1) - '0';
            int twoDigits = Integer.parseInt(s.substring(i - 2, i));

            if (oneDigit != 0) {
                dp[i] += dp[i - 1];
            }

            if (10 <= twoDigits && twoDigits <= 26) {
                dp[i] += dp[i - 2];
            }
        }

        return dp[n];
    }
}
// @lc code=end