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Hello xlog!

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use std::collections::VecDeque;

fn to_postfix(infix: &str) -> String {
let preced = |op: char| match op {
'(' | ')' => 1,
'^' => 2,
'*' | '/' => 3,
'+' | '-' => 4,
_ => unreachable!()
};
let op_left_assoc = |op: char| match op {
'^' => false,
_ => true
};
let mut stack: VecDeque<char> = VecDeque::new();
let mut output = String::new();
for x in infix.chars() {
match x {
ch if ch.is_digit(10) => output.push(ch),
'+' | '-' | '*' | '/' | '^' => {
while let Some(&op) = stack.back() {
if !(op != '(' && ((op_left_assoc(x) && preced(x) >= preced(op)) ||
(!op_left_assoc(x) && preced(x) > preced(op)))) {
break;
}

output.push(op);
stack.pop_back();
}
stack.push_back(x);
}
'(' => stack.push_back(x),
')' => {
while let Some(op) = stack.pop_back() {
if op == '(' { break; }
output.push(op);
}
}
_ => unreachable!()
}
}
output.push_str(stack.iter().rev()
.filter(|&&x| x != '(' && x != ')').collect::<String>().as_str());
output
}

// Add your tests here.
// See https://doc.rust-lang.org/stable/rust-by-example/testing/unit_testing.html

#[cfg(test)]
mod tests {
use itertools::Itertools;
use super::to_postfix;

fn do_test(actual: &str, expected: &str) {
assert_eq!(actual, expected, "\nYour answer (left) is not the correct answer (right)")
}

#[test]
fn fixed_tests() {
do_test(&to_postfix("2+7*5"), "275*+");
do_test(&to_postfix("3*3/(7+1)"), "33*71+/");
do_test(&to_postfix("5+(6-2)*9+3^(7-1)"), "562-9*+371-^+");
do_test(&to_postfix("(5-4-1)+9/5/2-7/1/7"), "54-1-95/2/+71/7/-");
do_test(&to_postfix("1^2^3"), "123^^");
}
}[]()