https://www.geeksforgeeks.org/problems/next-larger-element-1587115620/1?itm_source=geeksforgeeks&itm_medium=article&itm_campaign=practice_card

class Solution {
    // Function to find the next greater element for each element of the array.
    public ArrayList<Integer> nextLargerElement(int[] arr) {
        // code here
        ArrayList<Integer> al=new ArrayList<>();
        Stack<Integer> s=new Stack<>();
        for(int i=arr.length-1;i>=0;i--){
            int x=arr[i];
            while(!s.isEmpty() && x>=s.peek()){
                s.pop();
            }
            if(s.isEmpty()){
                al.add(-1);
            }else{
                al.add(s.peek());
            }
            //6 2 6
            //S= 6 
            //   6  -1
            //9 1 3 2 5
            //S=5 3 1
            //arr=-1  3  5  5  -1
            s.push(x);
        }
        Collections.reverse(al);
        return al;
        // int arr1[]=new int[al.size()];
        // int j=0;
        // for(Integer i:al){
        //     arr1[j]=i;
        //     j++;
        // }
        // return arr1;
    }
}
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https://www.geeksforgeeks.org/problems/smallest-number-on-left3403/1?itm_source=geeksforgeeks&itm_medium=article&itm_campaign=practice_card
class Solution{
    static List<Integer> leftSmaller(int n, int a[])
    {
        //code here
        Stack<Integer> stack=new Stack<>();
        ArrayList<Integer> al=new ArrayList<>();
        for(int i=0;i<n;i++){
            int x=arr[i];
            while(!stack.isEmpty() && x<stack.peek()){
                stack.pop();
            }
            if(stack.isEmpty()){
                al.add(-1);
            }else{
                al.add(stack.peek());
            }
            stack.push(x);
        }
        return al;
    }
}
//al//-1 1 1
//stack//1 
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