Create longest_substring_without_repeating_characters.cpp

longest_substring_without_repeating_characters.cpp

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+// Problem: Longest Substring Without Repeating Characters
+// LeetCode: https://leetcode.com/problems/longest-substring-without-repeating-characters/
+// Author: DeveloperViraj (curated DSA set for Hacktoberfest 2025)
+// Compile: g++ -std=c++17 01_longest_substring_without_repeating_characters.cpp -o 01_longest_substring
+// Run: ./01_longest_substring
+//
+// Summary:
+// Given a string, find the length of the longest substring without repeating characters.
+//
+// Approach (sliding window + last-seen index):
+// Use two indices (windowStart and windowEnd) and a map of last-seen indices for characters.
+// Expand the windowEnd and if the current character was seen inside the current window,
+// move windowStart to lastSeenIndex + 1. Update the maximum window length as you go.
+//
+// Time Complexity: O(n) where n = length of string (each char visited at most twice).
+// Space Complexity: O(min(n, charset)) for the hash map of last seen indices.
+
+#include <iostream>
+#include <string>
+#include <unordered_map>
+using namespace std;
+
+int lengthOfLongestUniqueSubstring(const string &s) {
+    unordered_map<char,int> lastSeen;
+    int maxLength = 0;
+    int windowStart = 0;
+
+    for (int windowEnd = 0; windowEnd < (int)s.size(); ++windowEnd) {
+        char currentChar = s[windowEnd];
+        if (lastSeen.find(currentChar) != lastSeen.end()) {
+            // If this character appeared inside current window, move start
+            if (lastSeen[currentChar] >= windowStart) {
+                windowStart = lastSeen[currentChar] + 1;
+            }
+        }
+        lastSeen[currentChar] = windowEnd;
+        int windowLength = windowEnd - windowStart + 1;
+        if (windowLength > maxLength) maxLength = windowLength;
+    }
+    return maxLength;
+}
+
+int main() {
+    cout << "Longest Substring Without Repeating Characters\n";
+    cout << "Enter a single-line string and press Enter:\n";
+    string input;
+    if (!getline(cin, input)) return 0;
+
+    int result = lengthOfLongestUniqueSubstring(input);
+    cout << "Length: " << result << "\n";
+    return 0;
+}
+
+/*
+Example:
+Input:
+abcabcbb
+Output:
+Length: 3  (substring "abc")
+*/