🟢 Port problem 136 (Single Number) from Swift to C++23

solutions/0101-0200/0136-single-number/README.md

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+# 136. Single Number
+
+[LeetCode problem 136](https://leetcode.com/problems/single-number/)
+
+- **Difficulty**: Easy
+- **Tags**: array, bit-manipulation
+
+## Description
+
+Given a non-empty array of integers where every element appears twice except
+for one, find that single element. Must run in O(n) time using O(1) extra
+space.

solutions/0101-0200/0136-single-number/solution.cpp

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+#include "solution.hpp"
+
+namespace leetcode::p0136 {
+
+int solve(const std::vector<int>& nums) {
+    unsigned result = 0;
+    for (int num : nums) {
+        result ^= static_cast<unsigned>(num);
+    }
+    return static_cast<int>(result);
+}
+
+} // namespace leetcode::p0136

solutions/0101-0200/0136-single-number/solution.hpp

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+#pragma once
+
+#include <vector>
+
+namespace leetcode::p0136 {
+
+// optimized: O(n) time, O(1) space — XOR cancels out every paired value.
+int solve(const std::vector<int>& nums);
+
+} // namespace leetcode::p0136

solutions/0101-0200/0136-single-number/test.cpp

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+#include <catch2/catch_test_macros.hpp>
+
+#include "solution.hpp"
+
+TEST_CASE("Problem 136", "[p0136]") {
+    CHECK(leetcode::p0136::solve({2, 2, 1}) == 1);
+    CHECK(leetcode::p0136::solve({4, 1, 2, 1, 2}) == 4);
+}