JSFiddle - React, Tailwind, and code Playground

by Mike Lin

JavaScript

function getNodeCount(rootValue, n) {
	let base = 10n
	let count = 1n
  while (BigInt(rootValue) * BigInt(base) <= BigInt(n)) {
  	// 如果本层级的最大数小于n, 那么节点数并不终结在本层级下，所以个数可以加上本层级的全部节点数
  	if (((BigInt(rootValue) + 1n) * BigInt(base)) - 1n < BigInt(n)) {
    	count = BigInt(count) + BigInt(base)
    } else {
    	count = BigInt(count) + BigInt(n) + 1n - BigInt(rootValue) * BigInt(base) 
    }
    base = BigInt(base) * 10n
  }
  
  return count
}

function lexicalOrder(n, m) {
	let ans = 1
  
  while(m !== 0) {
  	const count = getNodeCount(ans, n)
    console.log(count, m, ans)
    if (BigInt(count) >= BigInt(m)) {
    	m = BigInt(m) - 1n
      if (BigInt(m) === 0n) {
      	return ans
      }
      ans = BigInt(ans) * 10n
    } else {
    	m = BigInt(m) - BigInt(count)
      ans = BigInt(ans) + 1n
    }
  }
  
}

const m = 5555555555555555n

console.log(`字典序排在第${m}是：`, lexicalOrder(10000000000000000n, m))