Solution for "Search a 2D Matrix II"

Author: Thuy Trinh

src/main/java/leetcode/searchmatrix2/README.md

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+```
+[
+  [1,   4,  7, 11, 15],
+  [2,   5,  8, 12, 19],
+  [3,   6,  9, 16, 22],
+  [10, 13, 14, 17, 24],
+  [18, 21, 23, 26, 30]
+]
+```
+
+* Binary search on `[1, 4, 7, 11, 15]`:
+  * If we find the target, return true.
+  * Otherwise, return the index of max element which is smaller than the target. In this case, it's the index of `4`.
+* Binary search on `[2, 5, 8, 12, 19]` but the right bound is the index we retrieved from earlier step. That means `[2, 5]`. We find `5`, then return true.
+
+How binary search work?
+
+Ex: `[1, 4, 7, 11, 15]`, target is 5.
+
+middleIndex = (0 + 4) / 2 = 2, middle = 7. 7 > 5, so find on [1,4].

src/main/java/leetcode/searchmatrix2/Solution.kt

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+package leetcode.searchmatrix2
+
+sealed class Result
+object Found : Result()
+data class NotFoundYet(val maxIndex: Int) : Result()
+
+/**
+ * https://leetcode.com/problems/search-a-2d-matrix-ii/description/
+ */
+class Solution {
+  // a = [7, 8, 11, 15, 15], target = 12
+  private fun IntArray.bs(
+      left: Int = 0, // left = 3
+      right: Int = size - 1, // right = 2
+      target: Int // target = 12
+  ): Result {
+    if (left > right) {
+      return NotFoundYet(right)
+    }
+
+    val middle = (left + right) / 2 // middle = 3
+    val m = this[middle] // m = 15
+    return when {
+      m == target -> Found // false
+      m < target -> bs(middle + 1, right, target) // false
+      else -> bs(left, middle - 1, target) // true
+    }
+  }
+
+  fun searchMatrix(matrix: Array<IntArray>, target: Int): Boolean {
+    if (matrix.isEmpty() || matrix.first().isEmpty()) {
+      return false
+    }
+
+    var rightIndex = matrix.first().size - 1
+    matrix.takeWhile { it.first() <= target }
+        .forEach {
+          val r = it.bs(right = rightIndex, target = target)
+          when (r) {
+            is Found -> return true
+            is NotFoundYet -> rightIndex = r.maxIndex
+          }
+        }
+    return false
+  }
+}

src/test/java/leetcode/searchmatrix2/SolutionTest.kt

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+package leetcode.searchmatrix2
+
+import org.amshove.kluent.shouldBeFalse
+import org.amshove.kluent.shouldBeTrue
+import org.junit.Test
+
+class SolutionTest {
+  @Test
+  fun searchMatrix() {
+    Solution().searchMatrix(arrayOf(
+        intArrayOf(1, 4, 7, 11, 15),
+        intArrayOf(2, 5, 8, 12, 19),
+        intArrayOf(3, 6, 9, 16, 22),
+        intArrayOf(10, 13, 14, 17, 24),
+        intArrayOf(18, 21, 23, 26, 30)
+    ), target = 5).shouldBeTrue()
+    Solution().searchMatrix(arrayOf(
+        intArrayOf(3, 5, 9, 9, 14),
+        intArrayOf(7, 8, 11, 15, 15),
+        intArrayOf(8, 10, 16, 16, 17)
+    ), target = 12).shouldBeFalse()
+    Solution().searchMatrix(arrayOf(
+        intArrayOf(1)
+    ), target = 5).shouldBeFalse()
+    Solution().searchMatrix(arrayOf(
+        intArrayOf(2, 5, 8, 12, 19)
+    ), target = 19).shouldBeTrue()
+  }
+}