Tech Interview Handbook

https://www.techinterviewhandbook.org/
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### Python: Minimize Work Inside Loops for String Search

Source: https://www.techinterviewhandbook.org/coding-interview-techniques

This example optimizes the string search function by pre-calculating the lowercase version of the search term outside the loop. This avoids redundant computations within the loop, improving efficiency.

```Python
def contains_string(search_term, strings):
  for string in strings:
    if string.lower() == search_term.lower():
      return True
  return False
```

```Python
def contains_string(search_term, strings):
  search_term_lowercase = search_term.lower()
  for string in strings:
    if string.lower() == search_term_lowercase:
      return True
  return False
```

--------------------------------

### Implement Sequential Promise.all in JavaScript

Source: https://www.techinterviewhandbook.org/blog/a-glimpse-into-front-end-interviews

A JavaScript function to execute promises sequentially, ensuring each promise resolves before the next one starts. This is useful for scenarios where the order of operations matters.

```javascript
function sequential(data, fetcher) {  
  const helper = (index, results) => {  
    if (index === data.length) {  
      return results;
    }
    return fetcher(data[index]).then((datum) => {  
      results.push(datum);
      return helper(index + 1, results);
    });
  };

  return helper(0, []);
}

// usage example  
const fetcher = (i) => {  
  return new Promise((resolve) => {  
    setTimeout(() => resolve(i), 1000);
  });
};
sequential([1, 2, 3], fetcher);
```

--------------------------------

### Create an Empty N x M Matrix in Python

Source: https://www.techinterviewhandbook.org/algorithms/matrix

Initializes an N x M matrix with zeros. Assumes the input matrix is non-empty.

```python
zero_matrix = [[0 for _ in range(len(matrix[0]))] for _ in range(len(matrix))]
```

--------------------------------

### Merge Two Overlapping Intervals

Source: https://www.techinterviewhandbook.org/algorithms/interval

This function merges two overlapping intervals into a single interval that encompasses both. It calculates the minimum start point and the maximum end point of the two input intervals.

```python
def merge_overlapping_intervals(a, b):
  return [min(a[0], b[0]), max(a[1], b[1])]
```

--------------------------------

### Separate Pricing Data with Table-Driven Development in JavaScript

Source: https://www.techinterviewhandbook.org/blog/take-control-over-your-coding-interview

Use this pattern to externalize business rules like pricing into a separate configuration file. This makes the application logic cleaner and easier to update when business rules change. Ensure the data structure matches the lookup logic in the application file.

```javascript
// config.js
export const priceByRanges = [
  { min: 1, max: 5, price: '5' },
  { min: 6, max: 10, price: '4' },
  { min: 11, max: 20, price: '3' },
  { min: 21, max: Number.MAX_SAFE_INTEGER, price: '2.5' },
];

// app.js
import { priceByRanges } from './config.js';

function getPrice(amount) {
  // error handling for amount outside the range
  return priceByRanges.find(
    (priceByRange) => amount >= priceByRange.min && amount <= priceByRange.max,
  ).price;
}
```

--------------------------------

### Check if Two Intervals Overlap

Source: https://www.techinterviewhandbook.org/algorithms/interval

This function determines if two intervals, represented as two-element arrays [start, end], overlap. Ensure that the interval representation and overlap definition (inclusive/exclusive endpoints) are clarified with the interviewer.

```python
def is_overlap(a, b):
  return a[0] < b[1] and b[0] < a[1]
```

--------------------------------

### Optimize Price Lookup with Binary Search in JavaScript

Source: https://www.techinterviewhandbook.org/blog/take-control-over-your-coding-interview

When pricing data is sorted by ranges, leverage binary search for efficient lookups. This approach reduces the time complexity compared to a linear scan, especially for large datasets. Handle edge cases, such as amounts below the minimum range, explicitly.

```javascript
const priceByRanges = [
  { min: 1, max: 5, price: '5' },
  { min: 6, max: 10, price: '4' },
  { min: 11, max: 20, price: '3' },
  { min: 21, max: Number.MAX_SAFE_INTEGER, price: '2.5' },
];

function getPrice(amount) {
  if (amount < priceByRanges[0].min) {
    return 'unknown price';
  }

  let start = 0,
    end = priceByRanges.length - 1;

  while (start <= end) {
    const mid = (start + end) >>> 1;
    if (priceByRanges[mid].max < amount) {
      start = mid + 1;
    } else {
      end = mid - 1;
    }
  }

  return priceByRanges[start].price;
}
```

--------------------------------

### Get Price Per Unit Based on Purchase Amount

Source: https://www.techinterviewhandbook.org/blog/take-control-over-your-coding-interview

Use this function to determine the price per unit based on the quantity purchased. It handles different price tiers according to the provided table. Note that the last condition checks for amounts greater than 21, which might be a typo and should potentially be greater than 20.

```javascript
function getPrice(amount) {
  if (amount >= 1 && amount <= 5) return '5';
  if (amount >= 6 && amount <= 10) return '4';
  if (amount >= 11 && amount <= 20) return '3';
  if (amount > 21) return '2.5';
  return 'unknown price';
}
```

--------------------------------

### Breadth-First Search (BFS) on a Matrix

Source: https://www.techinterviewhandbook.org/algorithms/graph

A template for performing BFS on a matrix using a deque for efficient O(1) dequeuing. It explores nodes level by level.

```python
from collections import deque  

def bfs(matrix):
  # Check for an empty matrix/graph.  
  if not matrix:
    return []  
  
  rows, cols = len(matrix), len(matrix[0])  
  visited = set()  
  directions = ((0, 1), (0, -1), (1, 0), (-1, 0))  
  
  def traverse(i, j):
    queue = deque([(i, j)])  
    while queue:
      curr_i, curr_j = queue.popleft()
      if (curr_i, curr_j) not in visited:
        visited.add((curr_i, curr_j))
        # Traverse neighbors.  
        for direction in directions:
          next_i, next_j = curr_i + direction[0], curr_j + direction[1]
          if 0 <= next_i < rows and 0 <= next_j < cols:
            # Add in question-specific checks, where relevant.  
            queue.append((next_i, next_j))
  
  for i in range(rows):
    for j in range(cols):
      traverse(i, j)  

```

--------------------------------

### Depth-First Search (DFS) on a Matrix

Source: https://www.techinterviewhandbook.org/algorithms/graph

A template for performing DFS on a matrix. It uses a set to keep track of visited nodes and explores as far as possible along each branch before backtracking.

```python
def dfs(matrix):
  # Check for an empty matrix/graph.  
  if not matrix:
    return []  
  
  rows, cols = len(matrix), len(matrix[0])  
  visited = set()  
  directions = ((0, 1), (0, -1), (1, 0), (-1, 0))  
  
  def traverse(i, j):
    if (i, j) in visited:
      return  

    visited.add((i, j))
    # Traverse neighbors.  
    for direction in directions:
      next_i, next_j = i + direction[0], j + direction[1]
      if 0 <= next_i < rows and 0 <= next_j < cols:
        # Add in question-specific checks, where relevant.  
        traverse(next_i, next_j)  
  
  for i in range(rows):
    for j in range(cols):
      traverse(i, j)  

```

--------------------------------

### Implement Debounce and Throttle Functions in JavaScript

Source: https://www.techinterviewhandbook.org/blog/a-glimpse-into-front-end-interviews

Provides implementations for debounce and throttle functions, commonly asked in front end interviews. Debounce delays function execution until a certain time has passed without calls, while throttle ensures a function is called at most once within a specified interval.

```javascript
function debounce(fn, duration) {  
  let id;
  return function (...args) {  
    if (id) {  
      // reset timeout and prevent it from triggering  
      // if debounced function is called within duration  
      clearTimeout(id);
    }  
    id = setTimeout(() => {  
      fn(...args);
    }, duration);
  };
}

function throttle(fn, duration) {  
  let id;
  return function (...args) {  
    if (id) {  
      // if throttled function is called within duration,  
      // do nothing  
      return;
    }

    fn(...args);

    id = setTimeout(() => {  
      id = null; // release "lock"  
    }, duration);
  };
}

// usage example  
const helloWorld = () => {  
  console.log('hello world');
};
const debouncedHelloWorld = debounce(helloWorld, 1000);
const throttledHelloWorld = throttle(helloWorld, 1000);
```

--------------------------------

### Toggle the kth bit

Source: https://www.techinterviewhandbook.org/algorithms/binary

Use this snippet to toggle the k-th bit of a number (0 becomes 1, 1 becomes 0).

```Python
num ^= (1 << k)
```
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