Added task 2801

src/main/kotlin/g2801_2900/s2801_count_stepping_numbers_in_range/Solution.kt

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+package g2801_2900.s2801_count_stepping_numbers_in_range
+
+// #Hard #String #Dynamic_Programming #2024_01_19_Time_288_ms_(100.00%)_Space_38.2_MB_(100.00%)
+
+import kotlin.math.abs
+
+class Solution {
+    private lateinit var dp: Array<Array<Array<Array<Int?>>>>
+
+    fun countSteppingNumbers(low: String, high: String): Int {
+        dp = Array(low.length + 1) { Array(10) { Array(2) { arrayOfNulls(2) } } }
+        val count1 = solve(low, 0, 0, 1, 1)
+        dp = Array(high.length + 1) { Array(10) { Array(2) { arrayOfNulls(2) } } }
+        val count2 = solve(high, 0, 0, 1, 1)
+        return (count2!! - count1!! + isStep(low) + MOD) % MOD
+    }
+
+    private fun solve(s: String, i: Int, prevDigit: Int, hasBound: Int, curIsZero: Int): Int? {
+        if (i >= s.length) {
+            if (curIsZero == 1) {
+                return 0
+            }
+            return 1
+        }
+        if (dp[i][prevDigit][hasBound][curIsZero] != null) {
+            return dp[i][prevDigit][hasBound][curIsZero]
+        }
+        var count = 0
+        var limit = 9
+        if (hasBound == 1) {
+            limit = s[i].code - '0'.code
+        }
+        for (digit in 0..limit) {
+            val nextIsZero = if ((curIsZero == 1 && digit == 0)) 1 else 0
+            val nextHasBound = if ((hasBound == 1 && digit == limit)) 1 else 0
+            if (curIsZero == 1 || abs(digit - prevDigit) == 1) {
+                count = (count + solve(s, i + 1, digit, nextHasBound, nextIsZero)!!) % MOD
+            }
+        }
+        dp[i][prevDigit][hasBound][curIsZero] = count
+        return dp[i][prevDigit][hasBound][curIsZero]
+    }
+
+    private fun isStep(s: String): Int {
+        var isValid = true
+        for (i in 0 until s.length - 1) {
+            if (abs((s[i + 1].code - s[i].code)) != 1) {
+                isValid = false
+                break
+            }
+        }
+        if (isValid) {
+            return 1
+        }
+        return 0
+    }
+
+    companion object {
+        private const val MOD = (1e9 + 7).toInt()
+    }
+}

src/main/kotlin/g2801_2900/s2801_count_stepping_numbers_in_range/readme.md

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+2801\. Count Stepping Numbers in Range
+
+Hard
+
+Given two positive integers `low` and `high` represented as strings, find the count of **stepping numbers** in the inclusive range `[low, high]`.
+
+A **stepping number** is an integer such that all of its adjacent digits have an absolute difference of **exactly** `1`.
+
+Return _an integer denoting the count of stepping numbers in the inclusive range_ `[low, high]`_._
+
+Since the answer may be very large, return it **modulo** <code>10<sup>9</sup> + 7</code>.
+
+**Note:** A stepping number should not have a leading zero.
+
+**Example 1:**
+
+**Input:** low = "1", high = "11"
+
+**Output:** 10
+
+**Explanation:** The stepping numbers in the range [1,11] are 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10. There are a total of 10 stepping numbers in the range. Hence, the output is 10.
+
+**Example 2:**
+
+**Input:** low = "90", high = "101"
+
+**Output:** 2
+
+**Explanation:** The stepping numbers in the range [90,101] are 98 and 101. There are a total of 2 stepping numbers in the range. Hence, the output is 2.
+
+**Constraints:**
+
+*   <code>1 <= int(low) <= int(high) < 10<sup>100</sup></code>
+*   `1 <= low.length, high.length <= 100`
+*   `low` and `high` consist of only digits.
+*   `low` and `high` don't have any leading zeros.

src/test/kotlin/g2801_2900/s2801_count_stepping_numbers_in_range/SolutionTest.kt

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+package g2801_2900.s2801_count_stepping_numbers_in_range
+
+import org.hamcrest.CoreMatchers.equalTo
+import org.hamcrest.MatcherAssert.assertThat
+import org.junit.jupiter.api.Test
+
+internal class SolutionTest {
+    @Test
+    fun countSteppingNumbers() {
+        assertThat(Solution().countSteppingNumbers("1", "11"), equalTo(10))
+    }
+
+    @Test
+    fun countSteppingNumbers2() {
+        assertThat(Solution().countSteppingNumbers("90", "101"), equalTo(2))
+    }
+}