Off-by-one on range boundaries
Wrong move: Loop endpoints miss first/last candidate.
Usually fails on: Fails on minimal arrays and exact-boundary answers.
Fix: Re-derive loops from inclusive/exclusive ranges before coding.
Build confidence with an intuition-first walkthrough focused on core interview patterns fundamentals.
Table: Orders
+-----------------+----------+ | Column Name | Type | +-----------------+----------+ | order_number | int | | customer_number | int | +-----------------+----------+ order_number is the primary key (column with unique values) for this table. This table contains information about the order ID and the customer ID.
Write a solution to find the customer_number for the customer who has placed the largest number of orders.
The test cases are generated so that exactly one customer will have placed more orders than any other customer.
The result format is in the following example.
Example 1:
Input: Orders table: +--------------+-----------------+ | order_number | customer_number | +--------------+-----------------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 3 | +--------------+-----------------+ Output: +-----------------+ | customer_number | +-----------------+ | 3 | +-----------------+ Explanation: The customer with number 3 has two orders, which is greater than either customer 1 or 2 because each of them only has one order. So the result is customer_number 3.
Follow up: What if more than one customer has the largest number of orders, can you find all the customer_number in this case?
Problem summary: Table: Orders +-----------------+----------+ | Column Name | Type | +-----------------+----------+ | order_number | int | | customer_number | int | +-----------------+----------+ order_number is the primary key (column with unique values) for this table. This table contains information about the order ID and the customer ID. Write a solution to find the customer_number for the customer who has placed the largest number of orders. The test cases are generated so that exactly one customer will have placed more orders than any other customer. The result format is in the following example.
Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.
Pattern signal: General problem-solving
{"headers":{"orders":["order_number","customer_number"]},"rows":{"orders":[[1,1],[2,2],[3,3],[4,3]]}}Source-backed implementations are provided below for direct study and interview prep.
// Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
// Auto-generated Java example from rust.
class Solution {
public void exampleSolution() {
}
}
// Reference (rust):
// // Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
// pub fn sql_example() -> &'static str {
// r#"
// -- Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
// # Write your MySQL query statement below
// SELECT
// customer_number
// FROM orders
// GROUP BY customer_number
// ORDER BY COUNT(1) DESC
// LIMIT 1;
// "#
// }
// Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
// Auto-generated Go example from rust.
func exampleSolution() {
}
// Reference (rust):
// // Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
// pub fn sql_example() -> &'static str {
// r#"
// -- Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
// # Write your MySQL query statement below
// SELECT
// customer_number
// FROM orders
// GROUP BY customer_number
// ORDER BY COUNT(1) DESC
// LIMIT 1;
// "#
// }
# Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
# Auto-generated Python example from rust.
def example_solution() -> None:
return
# Reference (rust):
# // Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
# pub fn sql_example() -> &'static str {
# r#"
# -- Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
# # Write your MySQL query statement below
# SELECT
# customer_number
# FROM orders
# GROUP BY customer_number
# ORDER BY COUNT(1) DESC
# LIMIT 1;
# "#
# }
// Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
pub fn sql_example() -> &'static str {
r#"
-- Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
# Write your MySQL query statement below
SELECT
customer_number
FROM orders
GROUP BY customer_number
ORDER BY COUNT(1) DESC
LIMIT 1;
"#
}
// Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
// Auto-generated TypeScript example from rust.
function exampleSolution(): void {
}
// Reference (rust):
// // Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
// pub fn sql_example() -> &'static str {
// r#"
// -- Accepted solution for LeetCode #586: Customer Placing the Largest Number of Orders
// # Write your MySQL query statement below
// SELECT
// customer_number
// FROM orders
// GROUP BY customer_number
// ORDER BY COUNT(1) DESC
// LIMIT 1;
// "#
// }
Use this to step through a reusable interview workflow for this problem.
Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.
Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.
Review these before coding to avoid predictable interview regressions.
Wrong move: Loop endpoints miss first/last candidate.
Usually fails on: Fails on minimal arrays and exact-boundary answers.
Fix: Re-derive loops from inclusive/exclusive ranges before coding.