Leetcode #631: Design Excel Sum Formula
In this guide, we solve Leetcode #631 Design Excel Sum Formula in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Design the basic function of Excel and implement the function of the sum formula. Implement the Excel class: Excel(int height, char width) Initializes the object with the height and the width of the sheet.
Quick Facts
- Difficulty: Hard
- Premium: Yes
- Tags: Graph, Design, Topological Sort, Array, Hash Table, String, Matrix
Intuition
Fast membership checks and value lookups are the heart of this problem, which makes a hash map the natural choice.
By storing what we have already seen (or counts/indexes), we can answer the question in one pass without backtracking.
Approach
Scan the input once, using the map to detect when the condition is satisfied and to update state as you go.
This keeps the solution linear while remaining easy to explain in an interview setting.
Steps:
- Initialize a hash map for seen items or counts.
- Iterate through the input, querying/updating the map.
- Return the first valid result or the final computed value.
Example
Input
["Excel", "set", "sum", "set", "get"]
[[3, "C"], [1, "A", 2], [3, "C", ["A1", "A1:B2"]], [2, "B", 2], [3, "C"]]
Output
[null, null, 4, null, 6]
Explanation
Excel excel = new Excel(3, "C");
// construct a 3*3 2D array with all zero.
// A B C
// 1 0 0 0
// 2 0 0 0
// 3 0 0 0
excel.set(1, "A", 2);
// set mat[1]["A"] to be 2.
// A B C
// 1 2 0 0
// 2 0 0 0
// 3 0 0 0
excel.sum(3, "C", ["A1", "A1:B2"]); // return 4
// set mat[3]["C"] to be the sum of value at mat[1]["A"] and the values sum of the rectangle range whose top-left cell is mat[1]["A"] and bottom-right cell is mat[2]["B"].
// A B C
// 1 2 0 0
// 2 0 0 0
// 3 0 0 4
excel.set(2, "B", 2);
// set mat[2]["B"] to be 2. Note mat[3]["C"] should also be changed.
// A B C
// 1 2 0 0
// 2 0 2 0
// 3 0 0 6
excel.get(3, "C"); // return 6
Python Solution
Complexity
The time complexity is O(n). The space complexity is O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.