topic
Get all symmetric numbers (palindrome numbers) between 1-n, for example:
1-9
11 22 33 ..
101 111 121 131 ...
....
Idea one
Traverse, remember two variables n1, n2.
- n1 is the traversed number converted to a string
- n2 is to convert each number to a string, then to an array, then reverse, and then to a string.
Comparing n1 and n2 for equality is a palindrome.
/**
* 查询 1-max 的所有对称数（数组反转）
* @param max 最大值
*/
export function findPalindromeNumbers1(max: number): number[] {
const res: number[] = []
if (max <= 0) return res
for (let i = 1; i <= max; i++) {
// 转换为字符串，转换为数组，再反转，比较
const s = i.toString()
if (s === s.split('').reverse().join('')) {
res.push(i)
}
}
return res
}
Idea two
Similar to idea 1, idea 1 is converted to array comparison, and idea 2 compares strings from head to tail
/**
* 查询 1-max 的所有对称数（字符串前后比较）
* @param max 最大值
*/
export function findPalindromeNumbers2(max: number): number[] {
const res: number[] = []
if (max <= 0) return res
for (let i = 1; i <= max; i++) {
const s = i.toString()
const length = s.length
// 字符串头尾比较
let flag = true
let startIndex = 0 // 字符串开始
let endIndex = length - 1 // 字符串结束
while (startIndex < endIndex) {
if (s[startIndex] !== s[endIndex]) {
flag = false
break
} else {
// 继续比较
startIndex++
endIndex--
}
}
if (flag) res.push(i)
}
return res
}
Idea three
generate rollovers
/**
* 查询 1-max 的所有对称数（翻转数字）
* @param max 最大值
*/
export function findPalindromeNumbers3(max: number): number[] {
const res: number[] = []
if (max <= 0) return res
for (let i = 1; i <= max; i++) {
let n = i
let rev = 0 // 存储翻转数
// 生成翻转数
while (n > 0) {
rev = rev * 10 + n % 10
n = Math.floor(n / 10)
}
if (i === rev) res.push(i)
}
return res
}
test
// 功能测试
console.info(findPalindromeNumbers3(200))
// 性能测试
console.time('findPalindromeNumbers1')
findPalindromeNumbers1(100 * 10000)
console.timeEnd('findPalindromeNumbers1') // 408ms
console.time('findPalindromeNumbers2')
findPalindromeNumbers2(100 * 10000)
console.timeEnd('findPalindromeNumbers2') // 53ms
console.time('findPalindromeNumbers3')
findPalindromeNumbers3(100 * 10000)
console.timeEnd('findPalindromeNumbers3') // 42ms
- Array conversion takes time
- Manipulating numbers is faster, the prototype of the computer is the calculator