1375 - Number of Times Binary String Is Prefix-Aligned\.

Medium

You have a 1-indexed binary string of length n where all the bits are 0 initially. We will flip all the bits of this binary string (i.e., change them from 0 to 1) one by one. You are given a 1-indexed integer array flips where flips[i] indicates that the bit at index i will be flipped in the <code>i^th</code> step.

A binary string is prefix-aligned if, after the <code>i^th</code> step, all the bits in the inclusive range [1, i] are ones and all the other bits are zeros.

Return the number of times the binary string is prefix-aligned during the flipping process.

Example 1:

Input: flips = 3,2,4,1,5

Output: 2

Explanation: The binary string is initially "00000".

After applying step 1: The string becomes "00100", which is not prefix-aligned.

After applying step 2: The string becomes "01100", which is not prefix-aligned.

After applying step 3: The string becomes "01110", which is not prefix-aligned.

After applying step 4: The string becomes "11110", which is prefix-aligned.

After applying step 5: The string becomes "11111", which is prefix-aligned.

We can see that the string was prefix-aligned 2 times, so we return 2.

Example 2:

Input: flips = 4,1,2,3

Output: 1

Explanation: The binary string is initially "0000".

After applying step 1: The string becomes "0001", which is not prefix-aligned.

After applying step 2: The string becomes "1001", which is not prefix-aligned.

After applying step 3: The string becomes "1101", which is not prefix-aligned.

After applying step 4: The string becomes "1111", which is prefix-aligned.

We can see that the string was prefix-aligned 1 time, so we return 1.

Constraints:

• n == flips.length

• <code>1 <= n <= 5 * 10⁴</code>

• flips is a permutation of the integers in the range [1, n].