How to solve Leetcode 1695. Maximum Erasure Value
Another example of using unordered_map
Problem statement
You are given an array of positive integers nums and want to erase a subarray containing unique elements. The score you get by erasing the subarray is equal to the sum of its elements.
Return the maximum score you can get by erasing exactly one subarray.
An array b is called to be a subarray of a if it forms a contiguous subsequence of a, that is, if it is equal to a[l],a[l+1],...,a[r] for some (l,r).
Example 1
Input: nums = [4,2,4,5,6]
Output: 17
Explanation: The optimal subarray here is [2,4,5,6].
Example 2
Input: nums = [5,2,1,2,5,2,1,2,5]
Output: 8
Explanation: The optimal subarray here is [5,2,1] or [1,2,5].
Constraints
1 <= nums.length <= 10^5.1 <= nums[i] <= 10^4.
Solution: Store the position of the visited elements
You can use a map to store the position of the elements of nums. Then when iterating nums you can identify if an element has been visited before. That helps you to decide if a subarray contains unique elements.
Code
#include <iostream>
#include <unordered_map>
#include <vector>
using namespace std;
int maximumUniqueSubarray(vector<int>& nums) {
vector<int> sum(nums.size(), 0);
sum[0] = nums[0];
int maxSum = sum[0];
unordered_map<int, int> position;
position[nums[0]] = 0;
int start = -1;
for (int i = 1; i < nums.size(); i++) {
sum[i] = sum[i - 1] + nums[i];
if (position.find(nums[i]) != position.end()) {
start = max(start, position[nums[i]]);
}
position[nums[i]] = i;
maxSum = (start == -1) ? sum[i] : max(maxSum, sum[i] - sum[start]);
}
return maxSum;
}
int main() {
vector<int> nums{4,2,4,5,6};
cout << maximumUniqueSubarray(nums) << endl;
nums = {5,2,1,2,5,2,1,2,5};
cout << maximumUniqueSubarray(nums) << endl;
}
Output:
17
8
Complexity
Runtime:
O(N), whereN = nums.length.Extra space:
O(N).