LeetCode-in-Java
696. Count Binary Substrings
Easy
Give a binary string s, return the number of non-empty substrings that have the same number of 0’s and 1’s, and all the 0’s and all the 1’s in these substrings are grouped consecutively.
Substrings that occur multiple times are counted the number of times they occur.
Example 1:
Input: s = “00110011”
Output: 6
Explanation:
There are 6 substrings that have equal number of consecutive 1's and 0's: "0011", "01", "1100", "10", "0011", and "01".
Notice that some of these substrings repeat and are counted the number of times they occur.
Also, "00110011" is not a valid substring because all the 0's (and 1's) are not grouped together.
Example 2:
Input: s = “10101”
Output: 4
Explanation: There are 4 substrings: “10”, “01”, “10”, “01” that have equal number of consecutive 1’s and 0’s.
Constraints:
1 <= s.length <= 105s[i]is either'0'or'1'.
Solution
public class Solution {
public int countBinarySubstrings(String s) {
int start = 0;
int ans = 0;
char[] arr = s.toCharArray();
for (int i = 1; i < arr.length; i++) {
if (arr[i] != arr[i - 1]) {
ans++;
start = i - 1;
} else if (start > 0 && arr[--start] != arr[i]) {
// if start isn't 0, we may still have a valid substring
ans++;
} else {
// if not, then reset start to 0
start = 0;
}
}
return ans;
}
}