Replace Elements with Greatest Element on Right Side

#leetcode #dsa #theabbie

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

SOLUTION:

class Solution:
#     def makeSeg(self, arr, i, j):
#         seg = self.seg
#         if (i, j) in seg:
#             return seg[(i, j)]
#         if i == j:
#             seg[(i, j)] = arr[i]
#             return arr[i]
#         mid = (i + j) // 2
#         curr = max(self.makeSeg(arr, i, mid), self.makeSeg(arr, mid + 1, j))
#         seg[(i, j)] = curr
#         return curr

#     def getMax(self, arr, i, j, ni, nj):
#         seg = self.seg
#         if ni >= i and nj <= j:
#             return seg[(ni, nj)]
#         if (ni < i and nj < i) or (ni > j and nj > j):
#             return float('-inf')
#         mid = (ni + nj) // 2
#         return max(self.getMax(arr, i, j, ni, mid), self.getMax(arr, i, j, mid + 1, nj))

#     def replaceElements(self, arr: List[int]) -> List[int]:
#         n = len(arr)
#         self.seg = {}
#         self.makeSeg(arr, 0, n - 1)
#         for i in range(n):
#             if i < n - 1:
#                 arr[i] = self.getMax(arr, i + 1, n, 0, n - 1)
#             else:
#                 arr[i] = -1
#         return arr

    def replaceElements(self, arr: List[int]) -> List[int]:
        n = len(arr)
        op = [-1] * n
        currMax = float('-inf')
        for i in range(n - 1, 0, -1):
            currMax = max(currMax, arr[i])
            op[i - 1] = currMax
        return op