2981. Find Longest Special Substring That Occurs Thrice I

Author: sergeyleschev

2501-3000/2981. Find Longest Special Substring That Occurs Thrice I.ts

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+// Solution by Sergey Leschev
+// 2981. Find Longest Special Substring That Occurs Thrice I
+// Binary search
+
+function maximumLength(s: string): number {
+    const n = s.length
+
+    // Helper function to check if a special substring of length `x` exists at least thrice
+    const helper = (x: number): boolean => {
+        const count = new Array(26).fill(0)
+        let start = 0
+
+        for (let end = 0; end < n; end++) {
+            // Move start pointer until we match the current character
+            while (s[start] !== s[end]) start++
+            // Check if the substring length is at least `x`
+            if (end - start + 1 >= x) count[s.charCodeAt(end) - 'a'.charCodeAt(0)]++
+            // If any character count exceeds 2, return true
+            if (count[s.charCodeAt(end) - 'a'.charCodeAt(0)] > 2) return true
+        }
+
+        return false
+    }
+
+    let left = 1
+    let right = n
+
+    if (!helper(left)) return -1
+
+    while (left + 1 < right) {
+        const mid = Math.floor((left + right) / 2)
+        if (helper(mid)) {
+            left = mid
+        } else {
+            right = mid
+        }
+    }
+
+    return left
+}