1299.ReplaceElements.cs

// 1299. Replace Elements with Greatest Element on Right Side
// Given an array arr, replace every element in that array with the greatest element among the elements to its right, and replace the last element with -1.
// After doing so, return the array.

// Example 1:
// Input: arr = [17,18,5,4,6,1]
// Output: [18,6,6,6,1,-1]
// Explanation: 
// - index 0 --> the greatest element to the right of index 0 is index 1 (18).
// - index 1 --> the greatest element to the right of index 1 is index 4 (6).
// - index 2 --> the greatest element to the right of index 2 is index 4 (6).
// - index 3 --> the greatest element to the right of index 3 is index 4 (6).
// - index 4 --> the greatest element to the right of index 4 is index 5 (1).
// - index 5 --> there are no elements to the right of index 5, so we put -1.
// Example 2:
// Input: arr = [400]
// Output: [-1]
// Explanation: There are no elements to the right of index 0.

// Constraints:
// 1 <= arr.length <= 104
// 1 <= arr[i] <= 105

//Similar to Leader Election
public class Solution {
    public int[] ReplaceElements(int[] arr) {
        int[] result = new int[arr.Length];
        int leader = -1;
        for(int i = arr.Length-1; i>=0; i--){
            result[i] = leader;
            leader = Math.Max(leader, arr[i]);
        }
        return result;
    }
}