2960. Count Tested Devices After Test Operations 

Description

You are given a 0-indexed integer array batteryPercentages having length n, denoting the battery percentages of n 0-indexed devices.

Your task is to test each device i in order from 0 to n - 1, by performing the following test operations:

If batteryPercentages[i] is greater than 0:

<ul>
	<li><strong>Increment</strong> the count of tested devices.</li>
	<li><strong>Decrease</strong> the battery percentage of all devices with indices <code>j</code> in the range <code>[i + 1, n - 1]</code> by <code>1</code>, ensuring their battery percentage <strong>never goes below</strong> <code>0</code>, i.e, <code>batteryPercentages[j] = max(0, batteryPercentages[j] - 1)</code>.</li>
	<li>Move to the next device.</li>
</ul>
</li>
<li>Otherwise, move to the next device without performing any test.</li>

Return an integer denoting the number of devices that will be tested after performing the test operations in order.

Example 1:

Input: batteryPercentages = [1,1,2,1,3]
Output: 3
Explanation: Performing the test operations in order starting from device 0:
At device 0, batteryPercentages[0] > 0, so there is now 1 tested device, and batteryPercentages becomes [1,0,1,0,2].
At device 1, batteryPercentages[1] == 0, so we move to the next device without testing.
At device 2, batteryPercentages[2] > 0, so there are now 2 tested devices, and batteryPercentages becomes [1,0,1,0,1].
At device 3, batteryPercentages[3] == 0, so we move to the next device without testing.
At device 4, batteryPercentages[4] > 0, so there are now 3 tested devices, and batteryPercentages stays the same.
So, the answer is 3.

Example 2:

Input: batteryPercentages = [0,1,2]
Output: 2
Explanation: Performing the test operations in order starting from device 0:
At device 0, batteryPercentages[0] == 0, so we move to the next device without testing.
At device 1, batteryPercentages[1] > 0, so there is now 1 tested device, and batteryPercentages becomes [0,1,1].
At device 2, batteryPercentages[2] > 0, so there are now 2 tested devices, and batteryPercentages stays the same.
So, the answer is 2.

Constraints:

1 <= n == batteryPercentages.length <= 100
0 <= batteryPercentages[i] <= 100

Solutions

Solution 1: Simulation

Assume that the current number of devices we have tested is $ans$. When testing a new device $i$, its remaining battery is $\max(0, batteryPercentages[i] - ans)$. If the remaining battery is greater than $0$, it means this device can be tested, and we need to increase $ans$ by $1$.

Finally, return $ans$.

The time complexity is $O(n)$, where $n$ is the length of the array. The space complexity is $O(1)$.

Python3

class Solution:
    def countTestedDevices(self, batteryPercentages: List[int]) -> int:
        ans = 0
        for x in batteryPercentages:
            ans += x > ans
        return ans

Java

class Solution {
    public int countTestedDevices(int[] batteryPercentages) {
        int ans = 0;
        for (int x : batteryPercentages) {
            ans += x > ans ? 1 : 0;
        }
        return ans;
    }
}

C++

class Solution {
public:
    int countTestedDevices(vector<int>& batteryPercentages) {
        int ans = 0;
        for (int x : batteryPercentages) {
            ans += x > ans;
        }
        return ans;
    }
};

Go

func countTestedDevices(batteryPercentages []int) (ans int) {
	for _, x := range batteryPercentages {
		if x > ans {
			ans++
		}
	}
	return
}

TypeScript

function countTestedDevices(batteryPercentages: number[]): number {
    let ans = 0;
    for (const x of batteryPercentages) {
        ans += x > ans ? 1 : 0;
    }
    return ans;
}

Rust

impl Solution {
    pub fn count_tested_devices(battery_percentages: Vec<i32>) -> i32 {
        let mut ans = 0;
        for x in battery_percentages {
            ans += if x > ans { 1 } else { 0 };
        }
        ans
    }
}