LeetCode-Number of Digit One

LiBlog發表於2016-09-01

Given an integer n, count the total number of digit 1 appearing in all non-negative integers less than or equal to n.

For example:
Given n = 13,
Return 6, because digit 1 occurred in the following numbers: 1, 10, 11, 12, 13.

Analysis:

fullCount(i): how many 1 there are for 0...10^i-1, e.g. fullCount(2) is the number of 1 in [0...99]

Now, we need count the 1s in n digit by digit. Using an eample, n=456, we count the 1s in [0...400] (for digit 4) + [0...50] (for digit 5) + [0...6] (for digit 6).

For digit 4, which belongs to 10^i where i=3, there are 4*fullCount(2), i.e., 1s in [0...99] for 000, 100, 200, 300, + the 100 1s on the current digit for [100...199].

We can use similar way to calculate digit 5 and digit 6.

If digit x > 1, then

digitCount(i,x) = x*fullCount(i-1) + 10^(i-1);

Specially, if digit x==1 then

digitCount(i,x) = 1* fullCount(i-1) + n%10^(i-1) + 1.

e.g., n=156, for digit 1, the count is fullCount(2) for [0...99] + (56+1) for [100...156].

 

On the other hand, fullCount(i) = 10*fullCount(i-1) + 10^(i-1),

which directly equals to i*10^(i-1).

 

Solution:

 1 public class Solution {
 2     public int countDigitOne(int n) {
 3         int iter = 0                                        ;
 4         int fullCount = 0;
 5         long power10 = 1;
 6         int res = 0;
 7         while (n >= power10) {
 8             int digit = n / (int) power10 % 10;
 9             int digitCount = 0;
10             if (digit > 1) {
11                 digitCount = digit * fullCount + (int) power10;
12             } else if (digit == 1) {
13                 digitCount = fullCount + n % (int) power10 + 1;
14             }
15             res += digitCount;
16 
17             // Get iter, power10, full count.
18             // IMPORTANT: the sequence here is important!
19             // because fullCount(i) = i * 10^(i-1).
20             iter++;            
21             fullCount = iter * (int) power10;
22             power10 *= 10;
23         }
24         return res;
25     }
26 }

 

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