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Java-based LeetCode algorithm problem solutions, regularly updated
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package g2201_2300.s2266_count_number_of_texts;

// #Medium #String #Hash_Table #Dynamic_Programming #Math
// #2022_06_15_Time_38_ms_(81.43%)_Space_51.2_MB_(80.48%)

/**
 * 2266 - Count Number of Texts\.
 *
 * Medium
 *
 * Alice is texting Bob using her phone. The **mapping** of digits to letters is shown in the figure below.
 *
 * ![](https://assets.leetcode.com/uploads/2022/03/15/1200px-telephone-keypad2svg.png)
 *
 * In order to **add** a letter, Alice has to **press** the key of the corresponding digit `i` times, where `i` is the position of the letter in the key.
 *
 * *   For example, to add the letter `'s'`, Alice has to press `'7'` four times. Similarly, to add the letter `'k'`, Alice has to press `'5'` twice.
 * *   Note that the digits `'0'` and `'1'` do not map to any letters, so Alice **does not** use them.
 *
 * However, due to an error in transmission, Bob did not receive Alice's text message but received a **string of pressed keys** instead.
 *
 * *   For example, when Alice sent the message `"bob"`, Bob received the string `"2266622"`.
 *
 * Given a string `pressedKeys` representing the string received by Bob, return _the **total number of possible text messages** Alice could have sent_.
 *
 * Since the answer may be very large, return it **modulo** 10⁹ + 7.
 *
 * **Example 1:**
 *
 * **Input:** pressedKeys = "22233"
 *
 * **Output:** 8
 *
 * **Explanation:** 
 *
 * The possible text messages Alice could have sent are: 
 *
 * "aaadd", "abdd", "badd", "cdd", "aaae", "abe", "bae", and "ce". 
 *
 * Since there are 8 possible messages, we return 8.
 *
 * **Example 2:**
 *
 * **Input:** pressedKeys = "222222222222222222222222222222222222"
 *
 * **Output:** 82876089
 *
 * **Explanation:** There are 2082876103 possible text messages Alice could have sent. 
 *
 * Since we need to return the answer modulo 10⁹ + 7, we return 2082876103 % (10⁹ + 7) = 82876089.
 *
 * **Constraints:**
 *
 * *   1 <= pressedKeys.length <= 10⁵
 * *   `pressedKeys` only consists of digits from `'2'` - `'9'`.
**/
@SuppressWarnings("java:S135")
public class Solution {
    public int countTexts(String pressedKeys) {
        int len = pressedKeys.length();
        long dp0 = 1L;
        long dp1 = 0;
        long dp2 = 0;
        long dp3 = 0;
        long dp4;
        char[] keys = pressedKeys.toCharArray();
        int base = 1000000007;
        for (int i = 1; i < len; i++) {
            int r = keys[i] - '0';
            dp4 = dp3;
            dp3 = dp2;
            dp2 = dp1;
            dp1 = dp0 % base;
            dp0 = dp1;
            dp0 += (i - 1 == 0 && keys[i] == keys[i - 1]) ? 1 : 0;
            if (i - 1 <= 0 || keys[i] != keys[i - 1]) {
                continue;
            }
            dp0 += dp2;
            dp0 += (i - 2 == 0 && keys[i] == keys[i - 2]) ? 1 : 0;
            if (i - 2 <= 0 || keys[i] != keys[i - 2]) {
                continue;
            }
            dp0 += dp3;
            dp0 += (i - 3 == 0 && keys[i] == keys[i - 3] && (r == 7 || r == 9)) ? 1 : 0;
            if (i - 3 <= 0 || keys[i] != keys[i - 3] || (r != 7 && r != 9)) {
                continue;
            }
            dp0 += dp4;
        }

        return (int) (dp0 % base);
    }
}