38. Count and Say

Leetcode

https://leetcode.com/problems/count-and-say/

題目

The count-and-say sequence is a sequence of digit strings defined by the recursive formula:

• countAndSay(1) = "1"

• countAndSay(n) is the way you would "say" the digit string from countAndSay(n-1), which is then converted into a different digit string.

To determine how you "say" a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.

For example, the saying and conversion for digit string "3322251":

Given a positive integer n, return the nth term of the count-and-say sequence.

解答

• 方法一

遞迴計算

var countAndSay = function(n) {
    if(n===1) return '1';
    
    const str = countAndSay(n-1);
    let output = '';
    let count = 1;
    for(let i=0; i<str.length; i++) {
        if(str[i+1] === str[i]) count++;
        else {
            output += count.toString() + str[i];
            count = 1;
        }
    }
    
    return output;
};

Runtime: 72 ms, faster than 90.82% of JavaScript online submissions for Count and Say.

Memory Usage: 42 MB, less than 16.50% of JavaScript online submissions for Count and Say.

測資

console.log(countAndSay(5))

參考資料

https://www.itread01.com/content/1546736419.html