Make the array non-decreasing with the given operation

Given an array arr[] of size N, the task is to check if it is possible to make the array non-decreasing by applying the given operation at most once on each array element. In a single operation, one can decrease the element by one i.e. arr[i] = arr[i] – 1.
Examples:

Input: arr[] = {1, 2, 1, 2, 3} 
Output: Yes 
Apply the given operation on the 2^nd and the 4^th element. 
Now, the array becomes {1, 1, 1, 1, 3}
Input: arr[] = {1, 3, 1} 
Output: No 

Approach: Process the elements in increasing order and decrease the current element whenever it can be done without making it less than the previous element. (The first element should thus always be decreased.) If at any point the current element is less than the previous element then no matter what operation is performed, the answer is “No”.
The reason this strategy is optimal is that decreasing a number will make it easier (or at least easy) to deal with the next element in the list, since any value the next element could have taken will still work when the previous number is lower, and in fact decreasing the previous number expands the range of possible values for the next set.
Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

// Function to make array non-decreasing
bool isPossible(int a[], int n)
{
    // Take the first element
    int cur = a[0];

    // Perform the operation
    cur--;

    // Traverse the array
    for (int i = 1; i < n; i++) {

        // Next element
        int nxt = a[i];

        // If next element is greater than the
        // current element then decrease
        // it to increase the possibilities
        if (nxt > cur)
            nxt--;

        // It is not possible to make the
        // array non-decreasing with
        // the given operation
        else if (nxt < cur)
            return false;

        // Next element is now the current
        cur = nxt;
    }

    // The array can be made non-decreasing
    // with the given operation
    return true;
}

// Driver code
int main()
{
    int a[] = { 1, 2, 1, 2, 3 };
    int n = sizeof(a) / sizeof(a[0]);

    if (isPossible(a, n))
        cout << "Yes";
    else
        cout << "No";

    return 0;
}

Java

// Java implementation of the approach
class GFG
{

// Function to make array non-decreasing
static boolean isPossible(int a[], int n)
{
    // Take the first element
    int cur = a[0];

    // Perform the operation
    cur--;

    // Traverse the array
    for (int i = 1; i < n; i++)
    {

        // Next element
        int nxt = a[i];

        // If next element is greater than the
        // current element then decrease
        // it to increase the possibilities
        if (nxt > cur)
            nxt--;

        // It is not possible to make the
        // array non-decreasing with
        // the given operation
        else if (nxt < cur)
            return false;

        // Next element is now the current
        cur = nxt;
    }

    // The array can be made non-decreasing
    // with the given operation
    return true;
}

// Driver code
public static void main(String []args)
{
    int a[] = { 1, 2, 1, 2, 3 };
    int n = a.length;

    if (isPossible(a, n))
        System.out.printf("Yes");
    else
        System.out.printf("No");
}
}

// This code is contributed by 29AjayKumar

Python3

# Python3 implementation of the approach

# Function to make array non-decreasing
def isPossible(a, n) :

    # Take the first element
    cur = a[0];

    # Perform the operation
    cur -= 1;

    # Traverse the array
    for i in range(1, n) :

        # Next element
        nxt = a[i];

        # If next element is greater than the
        # current element then decrease
        # it to increase the possibilities
        if (nxt > cur) :
            nxt -= 1;

        # It is not possible to make the
        # array non-decreasing with
        # the given operation
        elif (nxt < cur) :
            return False;

        # Next element is now the current
        cur = nxt;

    # The array can be made non-decreasing
    # with the given operation
    return True;

# Driver code
if __name__ == "__main__" :

    a = [ 1, 2, 1, 2, 3 ];
    n = len(a);

    if (isPossible(a, n)) :
        print("Yes");
    else :
        print("No");

# This code is contributed by AnkitRai01

C#

// C# implementation of the approach
using System;

class GFG
{

// Function to make array non-decreasing
static Boolean isPossible(int []a, int n)
{
    // Take the first element
    int cur = a[0];

    // Perform the operation
    cur--;

    // Traverse the array
    for (int i = 1; i < n; i++)
    {

        // Next element
        int nxt = a[i];

        // If next element is greater than the
        // current element then decrease
        // it to increase the possibilities
        if (nxt > cur)
            nxt--;

        // It is not possible to make the
        // array non-decreasing with
        // the given operation
        else if (nxt < cur)
            return false;

        // Next element is now the current
        cur = nxt;
    }

    // The array can be made non-decreasing
    // with the given operation
    return true;
}

// Driver code
public static void Main(String []args)
{
    int []a = { 1, 2, 1, 2, 3 };
    int n = a.Length;

    if (isPossible(a, n))
        Console.WriteLine("Yes");
    else
        Console.WriteLine("No");
}
}

// This code is contributed by 29AjayKumar

Javascript

<script>

// JavaScript implementation of the approach

// Function to make array non-decreasing
function isPossible(a, n)
{

    // Take the first element
    var cur = a[0];

    // Perform the operation
    cur--;

    // Traverse the array
    for(var i = 1; i < n; i++)
    {

        // Next element
        var nxt = a[i];

        // If next element is greater than the
        // current element then decrease
        // it to increase the possibilities
        if (nxt > cur)
            nxt--;

        // It is not possible to make the
        // array non-decreasing with
        // the given operation
        else if (nxt < cur)
            return false;

        // Next element is now the current
        cur = nxt;
    }

    // The array can be made non-decreasing
    // with the given operation
    return true;
}

// Driver Code
var a = [ 1, 2, 1, 2, 3 ];
var n = a.length;

if (isPossible(a, n))
    document.write("Yes");
else
    document.write("No");

// This code is contributed by Khushboogoyal499

</script>

Output:

Yes

Time Complexity: O(n)

Auxiliary Space: O(1)