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Time: March 1 ~ March 13

  • Force buckle (LeetCode)- Sum of two numbers, valid parentheses, sum of two numbers | brush questions punch card – March 1
  • LeetCode – Merges two ordered lists, removes duplicates in sorted arrays,JavaScript Notes | Flash cards – March 2

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An array of

  1. Arrays are the simplest in-memory data structures
  2. An array stores a series of values of the same data type, or you can store values of different types in an array
  3. usepushMethod that can add elements to the end of an array. It can add any element
  4. useunshiftMethod to insert a value at the top of an array
  5. usepopMethod to delete the last element in the array
  6. useshiftDeletes the first element of the array

Add or remove elements at any location

Using the splice method, by specifying a location or index, you can remove elements of that location and number.

Example:

Numbers. The splice (5, 3); // drop numbers. Splice (5, 0, 1,2,3); // add the element numbers.splice(5, 3, 1,2,3) from index 5; // Three elements are removed from index 5Copy the code

2 d array

  • Matrix example:
Function printMatrix(myMatrix) {for (var I =0; i<myMatrix.length; i++){ for (var j=0; j<myMatrix[i].length; j++){ console.log(myMatrix[i][j]); }}}Copy the code
For (var I =0; i<matrix3x3x3.length; i++){ for (var j=0; j<matrix3x3x3[i].length; j++){ for (var z=0; z<matrix3x3x3[i][j].length; z++){ console.log(matrix3x3x3[i][j][z]); }}}Copy the code

Array methods (methods used by array structures and algorithms)

  1. concatJoins two or more arrays and returns the result
  2. every, runs the given function on each item in the array, returns true if the function returns true for each item
  3. filterReturns an array of items that return true by running the given function on each item in the array
  4. forEach, runs the given function on each item in the array. This method returns no value
  5. joinConcatenate all array elements into a string
  6. indexof, returns the index of the first array element equal to the given argument, or if no element is found- 1
  7. lastIndexOfReturns the maximum value in the index of the element found in the array equal to the given parameter
  8. mapReturns an array of the results of each function call by running the given function on each item in the array
  9. reverseReversing the order of the elements in the array, the first element is now the last, and the last element is now the present

Some, run the given function on each item in the array. If any item returns true, it returns true. 12. It is possible to pass a function specifying the sorting method as argument 13. toString, which returns an array as a string

The every method iterates through each element of the array until it returns false; The some method iterates through each element of the array until the function returns true

Example:

ForEach (function(ex){console.log((x % 2 == 0)); });Copy the code
  • reducemethods

The reduce method takes a function as a parameter, this function has four parameters: previousValue, currentValue, index and array.

Example:

numbers.reduce(function(previous,current,index){
 return previous + current;
});
Copy the code

New array methods in Es6 and Es7:

  • @@iteratorReturns an iterator object containing key-value pairs of array elements that can be called synchronously
  • copyWithinCopies a list of elements in an array to the starting position specified in the same array
  • entriesObject that contains all the key-value pairs of an array@@iterator
  • includesReturns true if there is an element in the array, false otherwise.ES7new
  • findRetrieves an element from the array based on the condition given by the callback function, and returns the element if found
  • findIndexRetrieves an element from the array based on the condition given by the callback function, and returns the element’s index in the array if found
  • fillPopulate the array with static values
  • fromCreates a new array based on the existing array
  • keysObject that contains all the indexes of an array@@iterator
  • ofCreates a new array based on the parameters passed in
  • valuesObject that contains all the values in the array@@iterator

Using the new ES6 iterator (@@iterator)

ES6 adds an @@iterator attribute to Array that needs to be accessed through symbol. iterator

Example:

let iterator = numbers[Symbol.iterator](); console.log(iterator.next().value); // 1 console.log(iterator.next().value); // 2 console.log(iterator.next().value); // 3 console.log(iterator.next().value); // 4 console.log(iterator.next().value); / / 5Copy the code

The Entries method returns an @@iterator containing key-value pairs

Example:

let aEntries = numbers.entries(); // Get iterator console.log(aendies.next ().value); // Get iterator console.log(aendies.next ().value); // [0, 1] - Position 0 with the value of 1 console.log(aendies.next ().value); // [1, 2] - Position 1 is 2 console.log(aendies.next ().value); // [2, 3] - Position 2 has a value of 3Copy the code

The keys method returns an @@iterator containing an array index

Example:

let aKeys = numbers.keys(); // Get the iterator for the array index console.log(akeys.next ()); // {value: 0, done: false } console.log(aKeys.next()); // {value: 1, done: false } console.log(aKeys.next()); // {value: 2, done: false }Copy the code

If the done attribute is false, it means there are still iterable values. Otherwise, vice versa.

The @@iterator returned by the values method contains the values of the array

Example:

let aValues = numbers.values(); 
console.log(aValues.next()); // {value: 1, done: false } 
console.log(aValues.next()); // {value: 2, done: false } 
console.log(aValues.next()); // {value: 3, done: false }
Copy the code

The array. from method creates a new Array from an existing Array

Example:

Let copyNumbers = array. from(numbers);Copy the code

Filter value function:

let dada = Array.from(numbers, x=> (x % 2 == 0));
Copy the code

The array. of method creates a new Array based on the parameters passed in

Example:

let da1 = Array.of(1); 
let da2 = Array.of(1, 2, 3, 4, 5, 6);

let da1 = [1]; 
let da2 = [1, 2, 3, 4, 5, 6];
Copy the code

Use this method to copy an existing array

Example:

let copyNumbers = Array.of(... da2);Copy the code

The fill method fills an array with static values

Example:

let numbersCopy = Array.of(1, 2, 3, 4, 5, 6); numbersCopy.fill(0); // [0, 0, 0, 0, 0, 0] numbersCopy.fill(2, 1); // [0, 2, 2, 2, 2, 2] let da = Array(6).fill(1); // [1, 1, 1, 1, 1]Copy the code

The copyWithin method copies a series of elements in an array to the starting position specified in the same array

Example:

let copyArray = [1, 2, 3, 4, 5, 6];
copyArray.copyWithin(0, 3);
// [4, 5, 6, 4, 5, 6]

copyArray = [1, 2, 3, 4, 5, 6]; 
copyArray.copyWithin(1, 3, 5);
// [1, 4, 5, 4, 5, 6]
Copy the code

search

  • indexOfMethod returns the index of the first element that matches the argument
  • lastIndexOfReturns the index of the last element that matches the argument

Differences between find and findIndex -ECMAScript 6

  • findMethod returns the first value that satisfies the condition; Can not find,findReturns theundefined
  • findIndexMethod returns the index of the value in the array, not found,findIndexReturn 1

Use the includes method -ecmascript 7

The toString and join

  • Prints all the elements in the array as a string

Example:

console.log(numbers.toString());

var numbersString = numbers.join('-'); 
console.log(numbersString);
Copy the code

53. Maximum suborder sum

1. Title Description

Given an integer array nums, find a contiguous subarray with the maximum sum (the subarray contains at least one element) and return the maximum sum.

Example 1: input: nums = [-2,1,-3,4,-1,2,1,-5,4] output: 6 explanation: the maximum sum of consecutive subarrays [4,-1,2,1] is 6. Example 2: Input: nums = [1] Output: 1 Example 3: Input: nums = [0] Output: 0 Example 4: Input: nums = [-1] Output: -1 Example 5: Input: nums = [-100000] Output: -100000Copy the code

Second, train of thought analysis

Method 1: Greedy algorithm: if the sum before the current pointer element is less than 0, the sequence before the current element is discarded

Answer code

/** * @param {number[]} nums * @return {number} */ var maxSubArray = function(nums) { let res = nums[0]; let sum = 0; for(let i=0; i<nums.length; i++){ if(sum>0) { sum += nums[i]; }else { sum = nums[i]; } res = Math.max(res,sum); } return res; };Copy the code

Four,

Maximum suborder sum – solution!

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