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2264 - Largest 3-Same-Digit Number in String (Easy)

https://leetcode.com/problems/largest-3-same-digit-number-in-string/

Problem Statement​

You are given a string num representing a large integer. An integer is good if it meets the following conditions:

  • It is a substring of num with length 3.
  • It consists of only one unique digit.

Return the maximum good integer as a string or an empty string "" if no such integer exists.

Note:

  • A substring is a contiguous sequence of characters within a string.
  • There may be leading zeroes in num or a good integer.

Example 1:

Input: num = "6777133339"
Output: "777"
Explanation: There are two distinct good integers: "777" and "333".
"777" is the largest, so we return "777".

Example 2:

Input: num = "2300019"
Output: "000"
Explanation: "000" is the only good integer.

Example 3:

Input: num = "42352338"
Output: ""
Explanation: No substring of length 3 consists of only one unique digit. Therefore, there are no good integers.

Constraints:

  • 3 <= num.length <= 1000
  • num only consists of digits.

Approach 1: Check the max digit​

Iterate num and check if the current character is same as the previous two. If so, store the maximum digit and render at the end.

Written by @wkw
class Solution {
public:
    string largestGoodInteger(string num) {
        int mx = -1;
        for (int i = 2; i < num.size(); i++) {
            // check if has one unique digit
            if (num[i] == num[i - 1] && num[i - 1] == num[i - 2]) {
                // if so, record the max digit
                mx = max(mx, num[i] - '0');
            }
        }
        // if mx is -1, then no such substring exists
        // otherwise, render the ans with the max digit
        return mx == -1 ? "" : string(3, mx + '0');
    }
};