feat(revision): add Day 03 - Stacks & Queues (Valid Parentheses, Next Greater Element)

CPP/algorithms/revision/25-revision-questions/day-03-stacks-queues/01_valid_parentheses.cpp

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+// 01_valid_parentheses.cpp
+// Problem: Valid Parentheses
+// LeetCode: https://leetcode.com/problems/valid-parentheses/
+// Author: DeveloperViraj (curated DSA set)
+// Compile: g++ -std=c++17 01_valid_parentheses.cpp -o 01_valid_parentheses
+// Run: ./01_valid_parentheses
+//
+// Description:
+//   Given a string containing only the characters '(', ')', '{', '}', '[' and ']',
+//   determine if the input string is valid. An input string is valid if:
+//     1) Open brackets are closed by the same type of brackets.
+//     2) Open brackets are closed in the correct order.
+//   The algorithm uses a stack to keep track of opening brackets; upon encountering
+//   a closing bracket we check whether the top of the stack matches the expected opening.
+//
+// Use case:
+//   Syntax validation, small parser checks, expression validation in editors.
+//
+// Time Complexity: O(n) — each character is pushed/popped at most once.
+// Space Complexity: O(n) — stack for open brackets (worst case all opens).
+//
+// Input format (example):
+//   A single line containing the bracket string.
+//   Example:
+//     ()[]{}         => Valid
+//     (]             => Invalid
+//
+// Output:
+//   Prints "Valid" if the string is valid, otherwise prints "Invalid".
+
+#include <iostream>
+#include <string>
+#include <stack>
+#include <unordered_map>
+
+using namespace std;
+
+bool isValidParentheses(const string &s) {
+    stack<char> openStack;
+    // Map closing -> corresponding opening
+    unordered_map<char, char> closingToOpening = {
+        {')', '('},
+        {'}', '{'},
+        {']', '['},
+    };
+
+    for (char ch : s) {
+        // If it's an opening bracket, push to stack
+        if (ch == '(' || ch == '{' || ch == '[') {
+            openStack.push(ch);
+        } else if (ch == ')' || ch == '}' || ch == ']') {
+            // If stack empty or top doesn't match expected opening, invalid
+            if (openStack.empty() || openStack.top() != closingToOpening[ch]) {
+                return false;
+            }
+            openStack.pop();
+        } else {
+            // If other characters exist, ignore or treat as invalid depending on spec.
+            // For this problem we assume input contains only parentheses characters.
+        }
+    }
+
+    // Valid only if no unmatched opens remain
+    return openStack.empty();
+}
+
+int main() {
+    cout << "🔹 Problem: Valid Parentheses\n";
+    cout << "Enter a single line containing the string of brackets (e.g. ()[]{}):\n";
+
+    string input;
+    if (!getline(cin, input)) {
+        cerr << "No input provided.\n";
+        return 0;
+    }
+
+    bool result = isValidParentheses(input);
+    if (result) cout << "Valid\n";
+    else cout << "Invalid\n";
+
+    return 0;
+}
+
+/*
+Example:
+Input:
+()[]{}
+
+Output:
+Valid
+*/

CPP/algorithms/revision/25-revision-questions/day-03-stacks-queues/02_next_greater_element.cpp

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+// 02_next_greater_element.cpp
+// Problem: Next Greater Element (monotonic stack)
+// LeetCode (related): https://leetcode.com/problems/next-greater-element-i/
+// Author: DeveloperViraj (curated DSA set)
+// Compile: g++ -std=c++17 02_next_greater_element.cpp -o 02_next_greater_element
+// Run: ./02_next_greater_element
+//
+// Description:
+//   For each element in an array, find the next greater element to its right.
+//   If no greater element exists, the answer for that position is -1.
+//   We use a monotonic decreasing stack that stores indices of elements whose
+//   next greater element hasn't been found yet. As we iterate, when we find
+//   a greater element, we pop and assign its next greater.
+//
+// Use case:
+//   Useful in many pattern problems: stock span variants, skyline queries,
+//   preprocessing for range queries, etc.
+//
+// Time Complexity: O(n) — each element is pushed and popped at most once.
+// Space Complexity: O(n) — stack + output array.
+//
+// Input format:
+//   n
+//   a0 a1 a2 ... a(n-1)
+//
+// Output:
+//   n integers (space-separated) — next greater element for each index, -1 if none.
+//
+// Example:
+// Input:
+// 9
+// 2 1 2 4 3 5 1 2 0
+//
+// Output:
+// 4 2 4 5 5 -1 2  -1 -1
+//
+// (Each index replaced by its next greater to the right)
+
+#include <iostream>
+#include <vector>
+#include <stack>
+
+using namespace std;
+
+vector<int> nextGreaterElements(const vector<int> &values) {
+    int n = (int)values.size();
+    vector<int> answer(n, -1);
+    stack<int> indexStack; // store indices of elements, monotonic decreasing by value
+
+    for (int i = 0; i < n; ++i) {
+        int currentValue = values[i];
+        // While current value is greater than the value at stack top index,
+        // we have found the next greater for that index.
+        while (!indexStack.empty() && values[indexStack.top()] < currentValue) {
+            int idx = indexStack.top();
+            indexStack.pop();
+            answer[idx] = currentValue;
+        }
+        // Push current index — its NG not known yet
+        indexStack.push(i);
+    }
+
+    // Remaining indices have no next greater; their answer remains -1.
+    return answer;
+}
+
+int main() {
+    cout << "🔹 Problem: Next Greater Element (to the right)\n";
+    cout << "Input format:\n n\n a0 a1 ... a(n-1)\nEnter n and the array:\n";
+
+    int n;
+    if (!(cin >> n) || n < 0) {
+        cerr << "Invalid input for n.\n";
+        return 0;
+    }
+
+    vector<int> arr(n);
+    for (int i = 0; i < n; ++i) cin >> arr[i];
+
+    vector<int> result = nextGreaterElements(arr);
+
+    cout << "\nNext greater elements:\n";
+    for (int i = 0; i < n; ++i) {
+        cout << result[i];
+        if (i + 1 < n) cout << ' ';
+    }
+    cout << '\n';
+
+    return 0;
+}
+
+/*
+Example:
+Input:
+6
+2 7 3 5 4 6
+
+Output:
+7 -1 5 6 6 -1
+
+Explanation:
+- Next greater after 2 is 7
+- 7 has no next greater -> -1
+- After 3 is 5
+- After 5 is 6
+- After 4 is 6
+- 6 has none -> -1
+*/