LeetCode #2693 — MEDIUM

Call Function with Custom Context

Move from brute-force thinking to an efficient approach using core interview patterns strategy.

The Problem

Problem Statement

Enhance all functions to have the callPolyfill method. The method accepts an object obj as its first parameter and any number of additional arguments. The obj becomes the this context for the function. The additional arguments are passed to the function (that the callPolyfill method belongs on).

For example if you had the function:

function tax(price, taxRate) {
  const totalCost = price * (1 + taxRate);
  console.log(`The cost of ${this.item} is ${totalCost}`);
}

Calling this function like tax(10, 0.1) will log "The cost of undefined is 11". This is because the this context was not defined.

However, calling the function like tax.callPolyfill({item: "salad"}, 10, 0.1) will log "The cost of salad is 11". The this context was appropriately set, and the function logged an appropriate output.

Please solve this without using the built-in Function.call method.

Example 1:

Input:
fn = function add(b) {
  return this.a + b;
}
args = [{"a": 5}, 7]
Output: 12
Explanation:
fn.callPolyfill({"a": 5}, 7); // 12
callPolyfill sets the "this" context to {"a": 5}. 7 is passed as an argument.

Example 2:

Input: 
fn = function tax(price, taxRate) { 
 return `The cost of the ${this.item} is ${price * taxRate}`; 
}
args = [{"item": "burger"}, 10, 1.1]
Output: "The cost of the burger is 11"
Explanation: callPolyfill sets the "this" context to {"item": "burger"}. 10 and 1.1 are passed as additional arguments.

Constraints:

typeof args[0] == 'object' and args[0] != null
1 <= args.length <= 100
2 <= JSON.stringify(args[0]).length <= 10⁵

Step 01

Brute Force Baseline

Problem summary: Enhance all functions to have the callPolyfill method. The method accepts an object obj as its first parameter and any number of additional arguments. The obj becomes the this context for the function. The additional arguments are passed to the function (that the callPolyfill method belongs on). For example if you had the function: function tax(price, taxRate) { const totalCost = price * (1 + taxRate); console.log(`The cost of ${this.item} is ${totalCost}`); } Calling this function like tax(10, 0.1) will log "The cost of undefined is 11". This is because the this context was not defined. However, calling the function like tax.callPolyfill({item: "salad"}, 10, 0.1) will log "The cost of salad is 11". The this context was appropriately set, and the function logged an appropriate output. Please solve this without using the built-in Function.call method.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: General problem-solving

Example 1

function add(b) { return this.a + b; }
[{"a":5},7]

Example 2

function tax(price, taxRate) { return `The cost of the ${this.item} is ${price * taxRate}`; }
[{"item":"burger"},10,1.1]

Step 02

Core Insight

What unlocks the optimal approach

No official hints in dataset. Start from constraints and look for a monotonic or reusable state.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #2693: Call Function with Custom Context
// Auto-generated Java example from ts.
class Solution {
    public void exampleSolution() {
    }
}
// Reference (ts):
// // Accepted solution for LeetCode #2693: Call Function with Custom Context
// declare global {
//     interface Function {
//         callPolyfill(context: Record<any, any>, ...args: any[]): any;
//     }
// }
// 
// Function.prototype.callPolyfill = function (context, ...args): any {
//     const fn = this.bind(context);
//     return fn(...args);
// };
// 
// /**
//  * function increment() { this.count++; return this.count; }
//  * increment.callPolyfill({count: 1}); // 2
//  */

// Accepted solution for LeetCode #2693: Call Function with Custom Context
// Auto-generated Go example from ts.
func exampleSolution() {
}
// Reference (ts):
// // Accepted solution for LeetCode #2693: Call Function with Custom Context
// declare global {
//     interface Function {
//         callPolyfill(context: Record<any, any>, ...args: any[]): any;
//     }
// }
// 
// Function.prototype.callPolyfill = function (context, ...args): any {
//     const fn = this.bind(context);
//     return fn(...args);
// };
// 
// /**
//  * function increment() { this.count++; return this.count; }
//  * increment.callPolyfill({count: 1}); // 2
//  */

# Accepted solution for LeetCode #2693: Call Function with Custom Context
# Auto-generated Python example from ts.
def example_solution() -> None:
    return
# Reference (ts):
# // Accepted solution for LeetCode #2693: Call Function with Custom Context
# declare global {
#     interface Function {
#         callPolyfill(context: Record<any, any>, ...args: any[]): any;
#     }
# }
# 
# Function.prototype.callPolyfill = function (context, ...args): any {
#     const fn = this.bind(context);
#     return fn(...args);
# };
# 
# /**
#  * function increment() { this.count++; return this.count; }
#  * increment.callPolyfill({count: 1}); // 2
#  */

// Accepted solution for LeetCode #2693: Call Function with Custom Context
// Rust example auto-generated from ts reference.
// Replace the signature and local types with the exact LeetCode harness for this problem.
impl Solution {
    pub fn rust_example() {
        // Port the logic from the reference block below.
    }
}

// Reference (ts):
// // Accepted solution for LeetCode #2693: Call Function with Custom Context
// declare global {
//     interface Function {
//         callPolyfill(context: Record<any, any>, ...args: any[]): any;
//     }
// }
// 
// Function.prototype.callPolyfill = function (context, ...args): any {
//     const fn = this.bind(context);
//     return fn(...args);
// };
// 
// /**
//  * function increment() { this.count++; return this.count; }
//  * increment.callPolyfill({count: 1}); // 2
//  */

// Accepted solution for LeetCode #2693: Call Function with Custom Context
declare global {
    interface Function {
        callPolyfill(context: Record<any, any>, ...args: any[]): any;
    }
}

Function.prototype.callPolyfill = function (context, ...args): any {
    const fn = this.bind(context);
    return fn(...args);
};

/**
 * function increment() { this.count++; return this.count; }
 * increment.callPolyfill({count: 1}); // 2
 */

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n)

Space

O(1)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

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.

OPTIMIZED

O(n) time

O(1) space

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.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

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.