Pinterest·Machine Learning Engineer·Technical Phone Screen·Senior

Senior

Jul 2026

Summary

Pinterest ML Engineer interview with a coding round that leaned more algorithmic than I expected. One problem stood out and I'm still thinking about whether my approach was actually clean or just lucky.

Questions Asked (1)

You're given an integer array called target. Starting from an all-zeros array of the same length, each operation lets you pick any subarray and increment every element in it by 1. What's the minimum number of operations needed to reach the target array?

Algorithms & Data Structures

Author's notes

The key is realizing each operation maps to a contiguous range of increments, so you're really counting how many times the value goes up from one index to the next.

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AI HintsAI Generated

Suggested Approach

Recognize that the minimum number of operations equals the sum of positive differences between consecutive elements in the target array, including the difference from 0 to the first element. This is because each operation can start a new 'layer' of increments, and the total layers needed is the sum of increases in height. Explain the intuition and provide a linear-time algorithm.

Pro tip: Connect the problem to real-world scenarios like image processing or resource allocation to show practical understanding, and mention that this is a classic problem often solved with a greedy approach.

1. Understand the problem

Restate the problem in your own words: we start with an array of zeros and each operation increments a contiguous subarray by 1. We need the minimum number of such operations to reach the target array.

2. Identify the key insight

The minimum operations equal the sum of positive differences between consecutive elements, including the difference from 0 to the first element. This is because each operation can only increase values, and the total increase needed is the sum of all 'upward steps'.

3. Derive the formula

Let target[0..n-1] be the array. Define target[-1] = 0. Then answer = sum_{i=0}^{n-1} max(0, target[i] - target[i-1]). Explain why this works: each operation contributes to the increase at some position, and the total number of operations is the total increase in the 'height profile'.

4. Provide an algorithm

Iterate through the array once, keeping track of the previous value (initialized to 0). For each element, if it's greater than the previous, add the difference to the total operations. Return the total. This runs in O(n) time and O(1) space.

5. Test with examples

Walk through a simple example, e.g., target = [1,2,3,2,1]. Compute the sum of positive differences: (1-0)+(2-1)+(3-2)+(2-3? no, negative so 0)+(1-2? no) = 1+1+1 = 3. Verify that 3 operations suffice: increment [0,2] by 1, then [1,2] by 1, then [2,2] by 1. This confirms the formula.

Key Points to Mention

The problem reduces to finding the sum of positive differences between consecutive elements.
Initialize the previous value to 0 to account for the first element.
Each operation can be thought of as adding a layer of 1s to a contiguous segment.
The algorithm runs in O(n) time and O(1) space.
The solution is optimal because each operation can at most increase the sum of positive differences by 1.
Edge cases: empty array (return 0), all zeros (return 0), strictly increasing array (answer is last element).

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.