PROBLEM DESCRIPTION
Given an array, find the nearest smaller element G[i] for every element A[i] in the array such that the element has an index smaller than i.
More formally,
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G[i] for an element A[i] = an element A[j] such that
j is maximum possible AND
j < i AND
A[j] < A[i]
Elements for which no smaller element exist, consider next smaller element as -1.
SOLUTION
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package com.gauk.stacks;
import java.util.Arrays;
import java.util.Stack;
public class NearestSmallest {
public static void main(String[] args) {
int[] a = new int[]{39, 27, 11, 4, 24, 32, 32, 1};
System.out.println(Arrays.toString(getNearestSmallerOnLeft(a)));
//Output: [-1, -1, -1, -1, 4, 24, -1, -1]
}
public static int[] getNearestSmallerOnLeft(int[] arr){
int[] out = new int[arr.length];
Arrays.fill(out, -1);
Stack<Integer> stack = new Stack<>();
stack.push(arr[0]);
for(int i=1; i<arr.length; i++){
while(!stack.isEmpty() && arr[i] <= stack.peek()){
stack.pop();
}
if(!stack.isEmpty() && arr[i] > stack.peek()){
out[i] = stack.peek();
}
stack.push(arr[i]);
}
return out;
}
}