Monotonic_Increasing_Stack

src/test/java/com/thealgorithms/stacks/MonotonicIncreasingStack.java

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+/**
+ * Contributor: Nayan Saraff
+ *
+ * A Monotonic Increasing Stack is an algorithmic pattern used to solve
+ * problems such as Stock Span, Trapping Rain Water, and Next Greater Element.
+ * It maintains a stack where elements are in increasing order to efficiently
+ * find relationships between elements based on their relative values.
+ *
+ * Reference:
+ * https://www.geeksforgeeks.org/dsa/introduction-to-monotonic-stack-2/
+ */
+
+package com.thealgorithms.stacks;
+
+import java.util.Stack;
+
+public final class MonotonicIncreasingStack {
+
+  private MonotonicIncreasingStack() {
+    throw new AssertionError("Cannot instantiate utility class");
+  }
+
+  /**
+   * Finds the next greater element for each element in the given array.
+   * For each element, it returns the nearest greater element to its right.
+   * If no such element exists, -1 is returned for that index.
+   *
+   * Time Complexity: O(n)
+   * Space Complexity: O(n)
+   */
+  public static int[] nextGreaterElement(int[] arr) {
+    int n = arr.length;
+    int[] result = new int[n];
+    Stack<Integer> stack = new Stack<>();
+
+    for (int i = n - 1; i >= 0; i--) {
+      while (!stack.isEmpty() && arr[i] >= arr[stack.peek()]) {
+        stack.pop();
+      }
+      result[i] = stack.isEmpty() ? -1 : arr[stack.peek()];
+      stack.push(i);
+    }
+    return result;
+  }
+
+  /**
+   * Finds the next smaller element for each element in the given array.
+   * For each element, it returns the nearest smaller element to its right.
+   * If no such element exists, -1 is returned for that index.
+   *
+   * Time Complexity: O(n)
+   * Space Complexity: O(n)
+   */
+  public static int[] nextSmallerElement(int[] arr) {
+    int n = arr.length;
+    int[] result = new int[n];
+    Stack<Integer> stack = new Stack<>();
+
+    for (int i = n - 1; i >= 0; i--) {
+      while (!stack.isEmpty() && arr[i] <= arr[stack.peek()]) {
+        stack.pop();
+      }
+      result[i] = stack.isEmpty() ? -1 : arr[stack.peek()];
+      stack.push(i);
+    }
+    return result;
+  }
+
+  // Test class included in the same file
+  public static void main(String[] args) {
+    testScenario(new int[]{2, 5, 1, 3, 4});
+    testScenario(new int[]{1, 2, 3, 4, 5});
+    testScenario(new int[]{5, 4, 3, 2, 1});
+  }
+
+  private static void testScenario(int[] arr) {
+    int[] nextGreater = nextGreaterElement(arr);
+    int[] nextSmaller = nextSmallerElement(arr);
+
+    System.out.print("Array: ");
+    printArray(arr);
+    System.out.print("Next Greater: ");
+    printArray(nextGreater);
+    System.out.print("Next Smaller: ");
+    printArray(nextSmaller);
+    System.out.println("-----------------------------");
+  }
+
+  private static void printArray(int[] arr) {
+    for (int n : arr) {
+      System.out.print(n + " ");
+    }
+    System.out.println();
+  }
+}