feat: solve 1087. Brace Expansion with unit testing (#95)

closes #86

Author: mathusanm6

README.md

@@ -15,8 +15,8 @@
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@@ -199,3 +199,9 @@ This repository covers a comprehensive range of algorithmic patterns and data st
 | # | Title | Solution | Time | Space | Difficulty | Tag | Note |
 |---|-------|----------|------|-------|------------|-----|------|
 | 2313 | [Minimum Flips in Binary Tree to Get Result](https://leetcode.com/problems/minimum-flips-in-binary-tree-to-get-result/) | [Python](./problems/minimum_flips_in_binary_tree_to_get_result/minimum_flips_in_binary_tree_to_get_result.py), [C++](./problems/minimum_flips_in_binary_tree_to_get_result/minimum_flips_in_binary_tree_to_get_result.cc) | _O(n)_ | _O(1)_ | Hard |  | _n_ is the number of nodes in the binary tree. |
+
+## Backtracking
+
+| # | Title | Solution | Time | Space | Difficulty | Tag | Note |
+|---|-------|----------|------|-------|------------|-----|------|
+| 1087 | [Brace Expansion](https://leetcode.com/problems/brace-expansion/) | [Python](./problems/brace_expansion/brace_expansion.py), [C++](./problems/brace_expansion/brace_expansion.cc) | _O(M^K + M log M)_ | _O(M^K)_ | Medium |  | M = max choices per brace set, K = number of brace sets. M^K for generating combinations, M log M for sorting. |

problems/brace_expansion/brace_expansion.cc

@@ -0,0 +1,85 @@
+#include "brace_expansion.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <regex>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace {
+
+std::vector<std::string> splitByComma(const std::string& str) {
+    std::vector<std::string> out;
+    std::size_t start = 0;
+    while (true) {
+        const std::size_t pos = str.find(',', start);
+        if (pos == std::string::npos) {
+            out.emplace_back(str.substr(start));
+            break;
+        }
+        out.emplace_back(str.substr(start, pos - start));
+        start = pos + 1;
+    }
+    return out;
+}
+
+}  // namespace
+
+std::vector<std::string> braceExpansion(const std::string& input) {
+    std::vector<std::string> prefixes;
+    prefixes.emplace_back("");  // Start with an empty prefix
+    std::string remaining = input;
+
+    // Regex captures (compile once; reuse):
+    // 1) leading literal before {   ([^{}]*)
+    // 2) choices inside {}          ([^}]*)
+    // 3) literal after }            ([^{}]*)
+    // 4) remaining tail             (.*)
+    static const std::regex REGEX(R"(([^{}]*)\{([^}]*)\}([^{}]*)(.*))");
+
+    std::smatch match;
+    // Iterate group by group until no more braces
+    while (std::regex_match(remaining, match, REGEX)) {
+        const std::string& lead = match[1].str();
+        const std::string& choicesStr = match[2].str();
+        const std::string& mid = match[3].str();
+        std::string tail = match[4].str();  // will become new remaining
+
+        // Expand choices and sort them
+        std::vector<std::string> choices = splitByComma(choicesStr);
+        if (choices.size() > 1) {
+            std::ranges::sort(choices);
+        }
+
+        std::vector<std::string> newPrefixes;
+        newPrefixes.reserve(prefixes.size() * std::max(choices.size(), 1UL));
+        for (const auto& prefix : prefixes) {
+            for (const auto& choice : choices) {
+                std::string newPrefix = prefix;
+                newPrefix += lead;
+                newPrefix += choice;
+                newPrefix += mid;
+                newPrefixes.emplace_back(std::move(newPrefix));
+            }
+        }
+
+        prefixes.swap(newPrefixes);  // reuse memory
+        remaining.swap(tail);        // continue with the tail
+    }
+
+    // Append final remaining tail (no more braces)
+    std::vector<std::string> results;
+    results.reserve(prefixes.size());
+    if (!prefixes.empty()) {
+        for (const auto& prefix : prefixes) {
+            std::string result = prefix;
+            result += remaining;
+            results.emplace_back(std::move(result));
+        }
+    } else {
+        results.emplace_back(remaining);  // input had no braces
+    }
+
+    return results;
+}

problems/brace_expansion/brace_expansion.h

@@ -0,0 +1,4 @@
+#include <string>
+#include <vector>
+
+std::vector<std::string> braceExpansion(const std::string& input);

problems/brace_expansion/brace_expansion.py

@@ -0,0 +1,53 @@
+import re
+
+from typing import List
+
+
+def braceExpansion(s: str) -> List[str]:
+    results: List[str] = []
+
+    def backtrack(prefixes: List[str], remaining: str) -> None:
+        """
+        Recursive helper that expands the string.
+
+        prefixes : partial expansions built so far
+        remaining: the remaining part of the string to process
+        """
+        nonlocal results
+
+        # Regex captures:
+        # 1) leading literal before {   ([^{}]*)
+        # 2) choices inside {}          ([^}]*)
+        # 3) literal after }            ([^{}]*)
+        # 4) remaining tail             (.*)
+        match = re.search(r"([^{}]*)\{([^}]*)\}([^{}]*)(.*)", remaining)
+
+        if not match:
+            # Base case: no more braces left
+            if prefixes:
+                # Append remaining to each existing prefix
+                results.extend(prefix + remaining for prefix in prefixes)
+            else:
+                # No prefix: just add the raw remaining string
+                results.append(remaining)
+            return
+
+        lead, choices_str, mid, tail = match.groups()
+        choices = sorted(choices_str.split(","))  # expand into sorted list of choices
+
+        # If no prefixes yet, start from empty string
+        base_prefixes = prefixes if prefixes else [""]
+        new_prefixes: List[str] = []
+
+        # Build new partial expansions by combining each prefix with each choice
+        for prefix in base_prefixes:
+            for choice in choices:
+                new_prefixes.append(prefix + lead + choice + mid)
+
+        # Recurse on the tail of the string
+        backtrack(new_prefixes, tail)
+
+    # Start recursion with empty prefix and full string
+    backtrack([], s)
+
+    return results

problems/brace_expansion/brace_expansion_test.cc

@@ -0,0 +1,40 @@
+#include "brace_expansion.h"
+
+#include <gtest/gtest.h>
+#include <string>
+#include <vector>
+
+struct BraceExpansionCase {
+    const std::string test_name;
+    const std::string input;
+    const std::vector<std::string> expected;
+};
+
+using BraceExpansionTest = ::testing::TestWithParam<BraceExpansionCase>;
+
+TEST_P(BraceExpansionTest, TestCases) {
+    const BraceExpansionCase &testCase = GetParam();
+    const auto result = braceExpansion(testCase.input);
+    EXPECT_EQ(result, testCase.expected);
+}
+
+INSTANTIATE_TEST_SUITE_P(BraceExpansionTestCases, BraceExpansionTest,
+                         ::testing::Values(
+                             BraceExpansionCase{
+                                 .test_name = "BasicCase",
+                                 .input = "{a,b}c{d,e}f",
+                                 .expected = {"acdf", "acef", "bcdf", "bcef"},
+                             },
+                             BraceExpansionCase{
+                                 .test_name = "NoBraces",
+                                 .input = "abcd",
+                                 .expected = {"abcd"},
+                             },
+                             BraceExpansionCase{
+                                 .test_name = "LeadingLiteralFollowedByBraces",
+                                 .input = "k{a,b,c,d,e,f,g}",
+                                 .expected = {"ka", "kb", "kc", "kd", "ke", "kf", "kg"},
+                             }),
+                         [](const testing::TestParamInfo<BraceExpansionCase> &info) {
+                             return info.param.test_name;
+                         });

problems/brace_expansion/brace_expansion_test.py

@@ -0,0 +1,25 @@
+"""Test cases for the brace_expansion function."""
+
+import pytest
+
+from brace_expansion import braceExpansion
+
+
+@pytest.mark.parametrize(
+    "s, expected",
+    [
+        ("{a,b}c{d,e}f", ["acdf", "acef", "bcdf", "bcef"]),  # Basic Case
+        ("abcd", ["abcd"]),  # No Braces
+        (
+            "k{a,b,c,d,e,f,g}",
+            ["ka", "kb", "kc", "kd", "ke", "kf", "kg"],
+        ),  # Leading Literal followed by Braces
+    ],
+    ids=[
+        "Basic Case",
+        "No Braces",
+        "Leading Literal followed by Braces",
+    ],
+)
+def test_brace_expansion(s, expected):
+    assert braceExpansion(s) == expected

problems/brace_expansion/config.yml

@@ -0,0 +1,17 @@
+problem:
+  number: 1087
+  title: "Brace Expansion"
+  leetcode_url: "https://leetcode.com/problems/brace-expansion/"
+  difficulty: "medium"
+  tags: ["Backtracking"]
+
+solutions:
+  python: "problems/brace_expansion/brace_expansion.py"
+  cpp: "problems/brace_expansion/brace_expansion.cc"
+
+complexity:
+  time: "O(M^K + M log M)"
+  space: "O(M^K)"
+
+notes: "M = max choices per brace set, K = number of brace sets. M^K for generating combinations, M log M for sorting."
+readme_link: ""

problems/calculate_amount_paid_in_taxes/calculate_amount_paid_in_taxes_test.cc

@@ -14,7 +14,7 @@ struct CalculateAmountPaidInTaxesCase {
 using CalculateAmountPaidInTaxesTest = ::testing::TestWithParam<CalculateAmountPaidInTaxesCase>;
 
 TEST_P(CalculateAmountPaidInTaxesTest, TestCases) {
-    CalculateAmountPaidInTaxesCase testCase = GetParam();
+    const CalculateAmountPaidInTaxesCase& testCase = GetParam();
     const auto result = calculateAmountPaidInTaxes(testCase.brackets, testCase.income);
     EXPECT_EQ(result, testCase.expected);
 }
@@ -33,6 +33,6 @@ INSTANTIATE_TEST_SUITE_P(
                                                      .brackets = {{2, 50}},
                                                      .income = 0,
                                                      .expected = 0.00000}),
-    [](const testing::TestParamInfo<CalculateAmountPaidInTaxesCase> &info) {
+    [](const testing::TestParamInfo<CalculateAmountPaidInTaxesCase>& info) {
         return info.param.test_name;
     });