400 Nth Digit
400. Nth Digit
1. Question
Find thenthdigit of the infinite integer sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ...
Note: nis positive and will fit within the range of a 32-bit signed integer (n< 231).
Example 1:
Input: 3
Output: 3
Example 2:
Input: 11
Output: 0
Explanation:
The 11th digit of the sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ... is a 0, which is part of the number 10.
2. Implementation
(1) Math
class Solution {
public int findNthDigit(int n) {
// 记录target number的长度
int digits = 1;
// 这里要用Long防止overflow
long count = 9;
int startNum = 1;
// Step 1: 找到第n个 digit所在的数的长度
while (n > count * digits) {
n -= count * digits;
digits += 1;
count *= 10;
startNum *= 10;
}
// Step 2:找到target number
int target = start + (n - 1) / digits; //减一是因为start本身算一个数
String s = Integer.toString(target); // 将target转成字符串方便下一步找到nth digit
// Step 3: (n - 1) % digits是nth digit在target number的位置
return Character.getNumericValue(s.charAt((n - 1) % digits));
}
}
3. Time & Space Complexity
时间复杂度O(logN), log的底是10, 空间复杂度O(1)
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