LeetCode 1130. Minimum Cost Tree From Leaf Values

Problem Statement

impl Solution {
    pub fn mct_from_leaf_values(arr: Vec<i32>) -> i32 {
        let (mut ans, n) = (0, arr.len());
        let mut stack = vec![];
        stack.push(std::i32::MAX);
        for a in &arr {
            while stack.last().unwrap() <= a {
                let mid = stack.last().unwrap().clone();
                stack.pop();
                ans += mid * std::cmp::min(stack.last().unwrap(), a);
            }
            stack.push(*a);
        }

        let mut i = 2;
        while i < stack.len() {
            ans += stack[i] * stack[i - 1];
            i += 1;
        }
        return ans;
    }
}