Largest number after mutating substring

Array String Greedy

37.9% acceptance

Feb 25, 2026

236

232

You are given a string num, which represents a large integer. You are also given a 0-indexed integer array change of length 10 that maps each digit 0-9 to another digit. More formally, digit d maps to digit change[d]. You may choose to mutate a single substring of num. To mutate a substring, replace each digit num[i] with the digit it maps to in change (i.e. replace num[i] with change[num[i]]). Return a string representing the largest possible integer after mutating (or choosing not to) a single substring of num. A substring is a contiguous sequence of characters within the string.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn maximum_number(num: String, change: Vec<i32>) -> String {
    let mut bytes: Vec<u8> = num.into_bytes();
    let mut started = false;
    for b in bytes.iter_mut() {
      let d = (*b - b'0') as usize;
      let mapped = change[d] as u8;
      if mapped > d as u8 {
        *b = b'0' + mapped;
        started = true;
      } else if mapped < d as u8 {
        if started {
          break;
        }
      }
      // if equal, continue (don't break the substring)
    }
    String::from_utf8(bytes).unwrap()
  }
}