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2802. Find The K-th Lucky Number

Description

We know that 4 and 7 are lucky digits. Also, a number is called lucky if it contains only lucky digits.

You are given an integer k, return the kth lucky number represented as a string.

 

Example 1:

Input: k = 4
Output: "47"
Explanation: The first lucky number is 4, the second one is 7, the third one is 44 and the fourth one is 47.

Example 2:

Input: k = 10
Output: "477"
Explanation: Here are lucky numbers sorted in increasing order:
4, 7, 44, 47, 74, 77, 444, 447, 474, 477. So the 10th lucky number is 477.

Example 3:

Input: k = 1000
Output: "777747447"
Explanation: It can be shown that the 1000th lucky number is 777747447.

 

Constraints:

  • 1 <= k <= 109

Solutions

  • class Solution {
        public String kthLuckyNumber(int k) {
            int n = 1;
            while (k > 1 << n) {
                k -= 1 << n;
                ++n;
            }
            StringBuilder ans = new StringBuilder();
            while (n-- > 0) {
                if (k <= 1 << n) {
                    ans.append('4');
                } else {
                    ans.append('7');
                    k -= 1 << n;
                }
            }
            return ans.toString();
        }
    }
    
  • class Solution {
    public:
        string kthLuckyNumber(int k) {
            int n = 1;
            while (k > 1 << n) {
                k -= 1 << n;
                ++n;
            }
            string ans;
            while (n--) {
                if (k <= 1 << n) {
                    ans.push_back('4');
                } else {
                    ans.push_back('7');
                    k -= 1 << n;
                }
            }
            return ans;
        }
    };
    
  • class Solution:
        def kthLuckyNumber(self, k: int) -> str:
            n = 1
            while k > 1 << n:
                k -= 1 << n
                n += 1
            ans = []
            while n:
                n -= 1
                if k <= 1 << n:
                    ans.append("4")
                else:
                    ans.append("7")
                    k -= 1 << n
            return "".join(ans)
    
    
  • func kthLuckyNumber(k int) string {
    	n := 1
    	for k > 1<<n {
    		k -= 1 << n
    		n++
    	}
    	ans := []byte{}
    	for n > 0 {
    		n--
    		if k <= 1<<n {
    			ans = append(ans, '4')
    		} else {
    			ans = append(ans, '7')
    			k -= 1 << n
    		}
    	}
    	return string(ans)
    }
    
  • function kthLuckyNumber(k: number): string {
        let n = 1;
        while (k > 1 << n) {
            k -= 1 << n;
            ++n;
        }
        const ans: string[] = [];
        while (n-- > 0) {
            if (k <= 1 << n) {
                ans.push('4');
            } else {
                ans.push('7');
                k -= 1 << n;
            }
        }
        return ans.join('');
    }
    
    

All Problems

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