JavaScript数组常见面试题

1. 数组清空

数组清空的三种方式

1. 修改length

let arr = [1,4,3,5,6]
console.log(arr)
arr.length = 0
console.log(arr)

2. splice

let arr = [5,6,7,8,9]
console.log(arr)
arr.splice(0)
console.log(arr)

3. 直接赋值

let arr = [9,5,3,3,2]
console.log(arr)
arr = []
console.log(arr)

2. 数组去重

去重的三种方式

1. 双重循环

var arr = [1,2,3,2,3,4,5,4,9,6,7,4,8,7] 
console.log(arr);
// 双重循环
var result = []
for (var i = 0; i < arr.length; i++) {
	for (var j = 0; j < result.length; j++) {
		if (result[j] === arr[i]) {
			break;
		}
	}
	
	if (j === result.length) {
		result.push(arr[i])
	}
}
console.log('arr = ', arr);
console.log('result = ', result);

时间复杂度：O(N^2)
0 + 1 + 2 + 3 + ... + arr.length-1 = N(N-1)/2$\approx$N^2

2. 数组排序去重

var arr = [1,2,3,2,3,4,5,4,9,6,7,4,8,7]
// 先排序
arr.sort()
var result = []
for (var i = 0; i < arr.length; i++) {
	// 因为arr是从小到大排序，所以只需判断result的栈顶元素是不是唯一的就可以压栈了
	if (result.length === 0 || result[result.length - 1] !== arr[i]) {
		result.push(arr[i])
	}
}
console.log('arr = ', arr)
console.log('resutl = ', result)

时间复杂度：O(N*logN)
arr(sort方法，堆排序)时间复杂度：O(N*logN)
生成result时间复杂度：O(N)

3. 哈希结构去重

var arr = [1,2,3,2,3,4,5,4,9,6,7,4,8,7]
var res = []
var arrMap = []
for (var i = 0; i < arr.length; i++) {
	arrMap[arr[i]] = true
}
for (var j in arrMap) {
	res.push(+j)
}
console.log('arr = ', arr)
console.log('arrMap = ', arrMap)
console.log('resutl = ', res)

时间复杂度：O(N)
主要取决于 arrMap 的时间复杂度