diff --git a/src/main/java/com/thealgorithms/datastructures/stacks/NearestElement.java b/src/main/java/com/thealgorithms/datastructures/stacks/NearestElement.java new file mode 100644 index 000000000000..465e13fccbe7 --- /dev/null +++ b/src/main/java/com/thealgorithms/datastructures/stacks/NearestElement.java @@ -0,0 +1,104 @@ +package datastructures.stacks; +import java.util.Arrays; +import java.util.Stack; + +/** + * NearestElement four classic stack based algorithms: + *Implemented methods: + * nearestGreaterToRight(int[] arr) - for each index i, first element to the right arr[i] or -1 + * nearestGreaterToLeft(int[] arr) - for each index i, first element to the left arr[i] or -1 + * nearestSmallerToRight(int[] arr) - for each index i, first element to the right arr[i] or -1 + * nearestSmallerToLeft(int[] arr) - for each index i, first element to the left arr[i] or -1 + * All methods run in O(n) time and O(n) extra space (stack + output). + * Usage: Each method returns an int[] of the same length as input. If no nearest element satisfies the condition, -1 is used at that index. + */ +public class NearestElement { + /** + * For each element, finds the nearest greater element to its right. + * Example: [4, 5, 2, 25] -> [5, 25, 25, -1] + * Time: O(n), Space: O(n) + */ + public static int[] nearestGreaterToRight(int[] arr) { + int n = arr.length; + int[] res = new int[n]; + Arrays.fill(res, -1); + Stack st = new Stack<>(); // it holds indices + + // traverse left to right, but we pop while current > stack top + for (int i = 0; i < n; i++) { + while (!st.isEmpty() && arr[i] > arr[st.peek()]) { + res[st.pop()] = arr[i]; + } + st.push(i); + } + return res; + } + + /** + * For each element, finds the nearest greater element to its left. + * Example: [4, 5, 2, 25] -> [-1, -1, 5, -1] + * Time: O(n), Space: O(n) + */ + public static int[] nearestGreaterToLeft(int[] arr) { + int n = arr.length; + int[] res = new int[n]; + Arrays.fill(res, -1); + Stack st = new Stack<>(); + for (int i = 0; i < n; i++) { + while (!st.isEmpty() && arr[st.peek()] <= arr[i]) { + st.pop(); + } + if (!st.isEmpty()) res[i] = arr[st.peek()]; + st.push(i); + } + return res; + } + + /** + * For each element, finds the nearest smaller element to its right. + * Example: [4, 5, 2, 25] -> [2, 2, -1, -1] + * Time: O(n), Space: O(n) + */ + public static int[] nearestSmallerToRight(int[] arr) { + int n = arr.length; + int[] res = new int[n]; + Arrays.fill(res, -1); + Stack st = new Stack<>(); + for (int i = 0; i < n; i++) { + while (!st.isEmpty() && arr[i] < arr[st.peek()]) { + res[st.pop()] = arr[i]; + } + st.push(i); + } + return res; + } + + /** + * For each element, finds the nearest smaller element to its left. + * Example: [4, 5, 2, 25] -> [-1, 4, -1, 2] + * Time: O(n), Space: O(n) + */ + public static int[] nearestSmallerToLeft(int[] arr) { + int n = arr.length; + int[] res = new int[n]; + Arrays.fill(res, -1); + Stack st = new Stack<>(); // holds indices + for (int i = 0; i < n; i++) { + while (!st.isEmpty() && arr[st.peek()] >= arr[i]) { + st.pop(); + } + if (!st.isEmpty()) res[i] = arr[st.peek()]; + st.push(i); + } + return res; + } + + public static void main(String[] args) { + int[] arr = {4, 5, 2, 25, 1, 7}; + System.out.println("Input: " + Arrays.toString(arr)); + System.out.println("Nearest Greater to Right: " + Arrays.toString(nearestGreaterToRight(arr))); + System.out.println("Nearest Greater to Left: " + Arrays.toString(nearestGreaterToLeft(arr))); + System.out.println("Nearest Smaller to Right: " + Arrays.toString(nearestSmallerToRight(arr))); + System.out.println("Nearest Smaller to Left: " + Arrays.toString(nearestSmallerToLeft(arr))); + } +}