## 439. Ternary Expression Parser

Given a string representing arbitrarily nested ternary expressions, calculate the result of the expression. You can always assume that the given expression is valid and only consists of digits `0-9`, `?`, `:`, `T` and `F` (`T` and `F` represent True and False respectively).

Note:

1. The length of the given string is ≤ 10000.
2. Each number will contain only one digit.
3. The conditional expressions group right-to-left (as usual in most languages).
4. The condition will always be either `T` or `F`. That is, the condition will never be a digit.
5. The result of the expression will always evaluate to either a digit `0-9`, `T` or `F`.

Example 1:

```Input: "T?2:3"

Output: "2"

Explanation: If true, then result is 2; otherwise result is 3.
```

Example 2:

```Input: "F?1:T?4:5"

Output: "4"

Explanation: The conditional expressions group right-to-left. Using parenthesis, it is read/evaluated as:

"(F ? 1 : (T ? 4 : 5))"                   "(F ? 1 : (T ? 4 : 5))"
-> "(F ? 1 : 4)"                 or       -> "(T ? 4 : 5)"
-> "4"                                    -> "4"
```

Example 3:

```Input: "T?T?F:5:3"

Output: "F"

Explanation: The conditional expressions group right-to-left. Using parenthesis, it is read/evaluated as:

"(T ? (T ? F : 5) : 3)"                   "(T ? (T ? F : 5) : 3)"
-> "(T ? F : 3)"                 or       -> "(T ? F : 5)"
-> "F"                                    -> "F"
```

## Rust Solution

``````struct Solution;

impl Solution {
fn parse_ternary(expression: String) -> String {
let expression: Vec<char> = expression.chars().collect();
Self::parse(&expression).iter().copied().collect()
}

fn parse(expression: &[char]) -> &[char] {
let n = expression.len();
if n == 1 {
expression
} else {
let mut count = 1;
for i in 2..n {
if expression[i] == '?' {
count += 1;
continue;
}
if expression[i] == ':' {
count -= 1;
if count == 0 {
return if expression == 'T' {
Self::parse(&expression[2..i])
} else {
Self::parse(&expression[i + 1..])
};
}
}
}
&[]
}
}
}

#[test]
fn test() {
let expression = "T?2:3".to_string();
let res = "2".to_string();
assert_eq!(Solution::parse_ternary(expression), res);
let expression = "F?1:T?4:5".to_string();
let res = "4".to_string();
assert_eq!(Solution::parse_ternary(expression), res);
let expression = "T?T?F:5:3".to_string();
let res = "F".to_string();
assert_eq!(Solution::parse_ternary(expression), res);
}
``````

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