fixed stacks example / closes #318

Author: Sobeston

website/versioned_docs/version-0.15.x/02-standard-library/11-stacks.md

@@ -0,0 +1,11 @@
+import CodeBlock from "@theme/CodeBlock";
+
+import Stacks from "!!raw-loader!./11.stacks.zig";
+
+# Stacks
+
+[`std.ArrayList`](https://ziglang.org/documentation/master/std/#std.ArrayList)
+provides the methods necessary to use it as a stack. Here's an example of
+creating a list of matched brackets.
+
+<CodeBlock language="zig">{Stacks}</CodeBlock>

website/versioned_docs/version-0.15.x/02-standard-library/11-stacks.mdx

@@ -0,0 +1,32 @@
+// hide-start
+const std = @import("std");
+const eql = std.mem.eql;
+const expect = std.testing.expect;
+const test_allocator = std.testing.allocator;
+// hide-end
+test "stack" {
+    const string = "(()())";
+    var stack: std.ArrayList(usize) = .empty;
+    defer stack.deinit(test_allocator);
+
+    const Pair = struct { open: usize, close: usize };
+    var pairs: std.ArrayList(Pair) = .empty;
+    defer pairs.deinit(test_allocator);
+
+    for (string, 0..) |char, i| {
+        if (char == '(') try stack.append(test_allocator, i);
+        if (char == ')')
+            try pairs.append(test_allocator, .{
+                .open = stack.pop() orelse
+                    @panic("mismatched brackets"),
+                .close = i,
+            });
+    }
+
+    const expected_pairs: []const Pair = &.{
+        .{ .open = 1, .close = 2 },
+        .{ .open = 3, .close = 4 },
+        .{ .open = 0, .close = 5 },
+    };
+    try std.testing.expectEqualDeep(expected_pairs, pairs.items);
+}