481 Magical String
481. Magical String
1. Question
A magical string S consists of only '1' and '2' and obeys the following rules:
The string S is magical because concatenating the number of contiguous occurrences of characters '1' and '2' generates the string S itself.
The first few elements of stringSis the following:S= "1221121221221121122……"
If we group the consecutive '1's and '2's inS, it will be:
1 22 11 2 1 22 1 22 11 2 11 22 ......
and the occurrences of '1's or '2's in each group are:
1 2 2 1 1 2 1 2 2 1 2 2 ......
You can see that the occurrence sequence above is theSitself.
Given an integer N as input, return the number of '1's in the first N number in the magical stringS.
Note:N will not exceed 100,000.
Example 1:
Input: 6
Output: 3
Explanation:
The first 6 elements of magical string S is "12211" and it contains three 1's, so return 3.
2. Implementation
(1) Two Pointers
思路: 这道题的关键在于找到形成magical string的规律,这里的规律是从"122"中的第3位开始,,该位置上是数字2,先形成2个1,然后到第四位,该位置上是数字1,形成1个2,然后第5位,该位置上是1, 形成1个1,以此类推
class Solution {
public int magicalString(int n) {
if (n <= 0) {
return 0;
}
if (n <= 3) {
return 1;
}
int[] sequence = new int[n + 1];
sequence[0] = 1;
sequence[1] = 2;
sequence[2] = 2;
int head = 2, tail = 3;
int res = 1;
int num = 1;
while (tail < n) {
for (int i = 0; i < sequence[head]; i++) {
sequence[tail] = num;
if (num == 1 && tail < n) {
++res;
}
++tail;
}
num ^= 3;
++head;
}
return res;
}
}
3. Time & Space Complexity
Two Pointers: 时间复杂度O(n), 空间复杂度O(n)
Last updated
Was this helpful?