Number of times binary string is prefix aligned

Array

66.0% acceptance

Feb 25, 2026

968

139

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 flips[i] will be flipped in the ith step. A binary string is prefix-aligned if, after the ith 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.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn num_times_all_blue(flips: Vec<i32>) -> i32 {
    let mut max_flip = 0i32;
    let mut count = 0;
    for (i, &f) in flips.iter().enumerate() {
      max_flip = max_flip.max(f);
      if max_flip == (i + 1) as i32 {
        count += 1;
      }
    }
    count
  }
}