Why use the comma-ok form for the map lookup?

A Go map returns the zero value for a missing key, so openerFor['('] quietly gives the byte 0 instead of failing. Writing opener, isCloser := openerFor[ch] adds a bool that says whether the key exists, which is the clearest way to ask whether ch is a closer at all. Testing opener != 0 would also work, since no bracket is the zero byte, but then correctness rests on that coincidence rather than on the map.

Could a switch statement replace the map?

Yes. Inside the loop, switch ch with cases for ), ] and } can set the wanted opener, and a default branch appends the byte and moves on. That skips hashing and keeps the three pairs next to the logic that uses them. The package-level map is shorter to extend if you later accept more pairs such as ; pick whichever reads better to you.

Should I loop over bytes or use for range to get runes?

Bytes are correct for this input. The six brackets are ASCII, and UTF-8 never places a byte below 0x80 inside a multi-byte character, so s[i] cannot confuse a piece of an emoji with (. A for _, r := range s loop decodes runes instead and works with a []rune stack and map[rune]rune, but that is decoding work with nothing gained. Also note that for i := range len(s) ranges over an integer, which needs Go 1.22 or newer; older toolchains need the three-part for i := 0; i < len(s); i++.

What happens if the string has letters or spaces in it?

As written, anything that is not a closer gets appended, so a letter lands on the stack, can never be matched, and the result is false. To ignore other bytes, append only when strings.IndexByte("([{", ch) >= 0, keep the closer branch as it is, and continue past everything else. Then isValid("total = f(x[0]) + {1: 2}[1]") is true and isValid("f(x[0)]") is still false.

How can I report where the string goes wrong instead of returning a bool?

Make the function return an int and keep a []int of opener positions, reading the opener byte back with s[stack[len(stack)-1]] when a closer arrives. A closer that meets an empty slice or the wrong opener returns its own index i. If the loop ends with positions left, the earliest unmatched opener is stack[0], the bottom of the slice; an empty slice means -1. For "[{]}", "(()[]", ")(" and "{[()()]}" that yields 2, 0, 0 and -1.

Valid Parentheses in Go (Golang): Pair Removal and Stack

Q: What happens if the string has letters or spaces in it?

As written, anything that is not a closer gets appended, so a letter lands on the stack, can never be matched, and the result is false. To ignore other bytes, append only when strings.IndexByte("([{", ch) >= 0, keep the closer branch as it is, and continue past everything else. Then isValid("total = f(x[0]) + {1: 2}[1]") is true and isValid("f(x[0)]") is still false.

is_valid.go

Go 1.26

Not run yet

+ 7 hidden

Input

s = "{[()()]}"

Expected

true

Each pair closes inside the one that opened before it, and nothing is left open.

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers

1 · Brute force: delete adjacent pairs until nothing changes

2 · One pass with a stack of expected closers