Create 2264-largest-3-same-digit-number-in-string.java

Author: sanajitjana

Easy/2264-largest-3-same-digit-number-in-string.java

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+/**
+ * Problem: 2264. Largest 3-Same-Digit Number in String
+ * Difficulty: Easy
+ * URL: https://leetcode.com/problems/largest-3-same-digit-number-in-string/
+ *
+ * ---------------------
+ * Approach 1: Brute Force (Substring + Parse)
+ * ---------------------
+ * - Iterate through the string and check if three consecutive characters are the same.
+ * - If so, parse them as a number and track the maximum.
+ * 
+ * Time Complexity:  O(n)
+ * Space Complexity: O(1)
+ */
+class Solution {
+    public String largestGoodInteger(String num) {
+        long max = Long.MIN_VALUE;
+        for (int i = 0; i < num.length() - 2; i++) {
+            if (num.charAt(i) == num.charAt(i + 1) &&
+                num.charAt(i + 1) == num.charAt(i + 2)) {
+                
+                long val = Long.parseLong(num.substring(i, i + 3));
+                if (val > max) max = val;
+            }
+        }
+        if (max == 0) return "000";
+        return max > Long.MIN_VALUE ? String.valueOf(max) : "";
+    }
+}
+
+/**
+ * ---------------------
+ * Approach 2: Brute Force with Counting
+ * ---------------------
+ * - Iterate through the string with a counter for consecutive identical digits.
+ * - When count reaches 3, update the max digit.
+ * - Avoid parsing; directly compare characters for efficiency.
+ * 
+ * Time Complexity:  O(n)
+ * Space Complexity: O(1)
+ */
+class Solution {
+    public String largestGoodInteger(String num) {
+        int count = 0;
+        char max = ' ';
+        char prev = ' ';
+        
+        for (char ch : num.toCharArray()) {
+            if (ch == prev) count++;
+            else count = 1;
+            
+            if (count == 3) {
+                max = (char) Math.max(max, ch);
+            }
+            prev = ch;
+        }
+        return max == ' ' ? "" : String.valueOf(max).repeat(3);
+    }
+}
+
+/**
+ * ---------------------
+ * Approach 3: Optimal (Predefined Candidates)
+ * ---------------------
+ * - Pre-store all possible valid 3-digit identical numbers from largest to smallest.
+ * - Check if the input contains each one; return the first match.
+ * - Since the array is constant size (10 elements), this is very fast.
+ * 
+ * Time Complexity:  O(n)   [10 × O(n) => O(n)]
+ * Space Complexity: O(1)
+ */
+class Solution {
+    public String largestGoodInteger(String num) {
+        String[] candidates = { "999", "888", "777", "666", "555", 
+                                 "444", "333", "222", "111", "000" };
+        for (String c : candidates) {
+            if (num.contains(c)) return c;
+        }
+        return "";
+    }
+}