# Interesting Python Implementation for Next Greater Elements

Given an array, print the Next Greater Element (NGE) for every element. The Next greater Element for an element x is the first greater element on the right side of x in array. Elements for which no greater element exist, consider next greater element as -1.

Examples:

```Input : 11, 13, 21, 3, 9, 12
Output :
11 --> 21
13 --> 21
21 --> -1
3 --> 12
9 --> 12
12 --> -1

Input : 10, 9, 8, 7, 6, 5, 4, 3, 2
Output :
10 --> -1
9 --> -1
8 --> -1
7 --> -1
6 --> -1
5 --> -1
4 --> -1
3 --> -1
2 --> -1
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Below is a simple implementation in Python.

 `# Function for implementation of NGE ` `def` `NGE(arr): ` ` `  `    ``# Iterate through array to check for greatest element ` `    ``for` `i ``in` `range``(``0``, ``len``(arr)): ` ` `  `        ``# Find the maximum from i to end ` `        ``lead ``=` `max``(arr[i:]) ` ` `  `        ``# If max is same as the i'th number then  ` `        ``# print -1 else print the maximum ` `        ``if` `(arr[i] ``=``=` `lead): ` `            ``print``(``"% d --> % d"` `%` `(arr[i], ``-``1``)) ` `        ``else``: ` `            ``print``(``"% d --> % d"` `%` `(arr[i], lead)) ` ` `  ` `  `# Driver program ` `def` `main(): ` `    ``arr ``=` `[``11``, ``13``, ``21``, ``3``, ``9``, ``12``] ` `    ``NGE(arr) ` `    ``arr ``=` `[``10``, ``9``, ``8``, ``7``, ``6``, ``5``, ``4``, ``3``, ``2``] ` `    ``NGE(arr) ` ` `  `if` `__name__ ``=``=` `'__main__'``: ` `    ``main() `

Note that the time complexity of above solution is O(n*n). We can optimize it in O(n) time using stack.

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