first questions

`Array.prototype.indexOf` returns the index of the first occurrence of a specified element within the array. If the element is not found, it returns `-1`. It performs a strict comparison (===). 

const arr = ['a', 'b', 'c'];
console.log(arr.indexOf('b')); // 1
console.log(arr.indexOf('d')); // -1

`String.prototype.indexOf` returns the index of the first occurrence of a specified value in a string. If the value is not found, it returns -1. 

const str = 'hello';
const index = str.indexOf('l');
console.log(index); // 2

`String.prototype.replace` replaces the first occurrence of a substring or pattern (regular expression) with a new substring. To replace all occurrences, a global regular expression must be used. 

const str = 'hello world';
const newStr = str.replace('world', 'JavaScript');
console.log(newStr); // 'hello JavaScript'

`Array.prototype.find` returns the first element in the array that satisfies the provided testing function. If no elements satisfy the function, it returns `undefined`. It does not modify the original array. 

const arr = [1, 2, 3];
const firstEven = arr.find(num => num % 2 === 0);
console.log(firstEven); // 2

`Array.prototype.shift` removes the first element from an array and returns that element. It modifies the original array and shifts all subsequent elements down by one. 

const arr = [1, 2, 3];
const first = arr.shift();
console.log(first); // 1
console.log(arr); // [2, 3]

`String.prototype.search` searches a string for a match against a regular expression and returns the index of the first match. If no match is found, it returns -1. 

const str = 'hello 123'; 
const index = str.search(/\d+/); 
console.log(index); // 6 

`Array.prototype.flatMap` maps each element using a provided mapping function and then flattens the resulting array into a new array. It combines the map and flat operations into a single method. 

const arr = [1, 2, 3];
const flatMapArr = arr.flatMap(x => [x, x * 2]);
console.log(flatMapArr); // [1, 2, 2, 4, 3, 6]

`Array.prototype.findIndex` returns the index of the first element in the array that satisfies the provided testing function. If no elements satisfy the function, it returns `-1`. It does not modify the original array. 

const arr = [5, 12, 8];
const index = arr.findIndex(num => num > 10);
console.log(index); // 1

The VLOOKUP function searches for a value in the first column of a table and returns a value from a specified column in the same row. For example, =VLOOKUP('Apple', A1:C10, 2, FALSE) searches for 'Apple' in column A and returns the corresponding value from column B. This function is useful for looking up information in tables.

DFS explores as far as possible along each branch before backtracking. Use a stack or recursion to keep track of nodes. For example, in a graph with nodes 1 -> 2 -> 3, DFS from 1 explores 1, 2, and then 3.

BFS explores all neighbors of a node before moving to the next level. Use a queue to keep track of nodes. For example, in a graph with nodes 1 -> 2 -> 3, BFS from 1 explores 1, then 2 and 3.

Use a hash map to count the frequency of each character, then iterate through the string to find the first character with a count of 1. For example, in 'swiss', the first non-repeating character is 'w'.