检查 usize 中整数溢出的规范方法

Question

我试图在 Rust 中解决以下 leetcode problem：

impl Solution {
    pub fn merge(nums1: &mut Vec<i32>, m: i32, nums2: &mut Vec<i32>, n: i32) {
        let mut backwards_idx: usize = (m + n - 1) as usize;
        let (m_size, n_size) = (m as usize, n as usize);
        let (mut m_idx, mut n_idx) = ((m-1) as usize, (n-1) as usize);
        while 0 <= m_idx && 0 <= n_idx {
            let (nums1_elem, nums2_elem) = (nums1[m_idx], nums2[n_idx]);
            if nums1_elem <= nums2_elem {
                nums1[backwards_idx] = nums2_elem;
                n_idx = n_idx - 1;
            } else {
                nums1[backwards_idx] = nums1_elem;
                m_idx = m_idx - 1;
            }
            backwards_idx = backwards_idx - 1;
        }
        while 0 <= m_idx {
            nums1[backwards_idx] = nums1[m_idx];
            m_idx = m_idx - 1;
            backwards_idx = backwards_idx - 1;
        }
        while 0 <= n_idx {
            nums1[backwards_idx] = nums2[n_idx];
            n_idx = n_idx - 1;
            backwards_idx = backwards_idx - 1;
        }
    }
}

但是，这行不通，因为从 0 中减去 m_size 和 n_size 会溢出。我知道这不是规范的 Rust，但由于问题在 i32，我改不了多少。

有，但它似乎很难阅读，而不是直接在 while 循环条件下写入它。在 while 循环体中设置退出条件似乎不是个好主意。

Answer 1

一种避免溢出的方法是用不同的方式构造循环.... Rust 没有明确的 do-while 语法，但它确实有 loop 你可以 break 使用标签从，甚至突破到任意数量的外循环。

    loop {
        nums1[backwards_idx] = nums1[m_idx];
        if m_idx == 0 { break; }
        m_idx -= 1;
        backwards_idx -= 1;
    }

Answer 2

避免必须检查溢出的一个简单方法是保证您的 usize 类型，它们只会被减 1，从不为 0在这些循环中开始。将 idx 类型重构为计数，并在我们想要获得索引时减去 1，这是一种方法...

impl Solution {
    pub fn merge(nums1: &mut Vec<i32>, m: i32, nums2: &mut Vec<i32>, n: i32) {
        let mut numbers_left: usize = (m + n) as usize;
        let (mut m_left, mut n_left) = (m as usize, n as usize);
        while m_left > 0 && n_left > 0 {
            let (nums1_elem, nums2_elem) = (nums1[m_left-1], nums2[n_left-1]);
            if nums1_elem <= nums2_elem {
                nums1[numbers_left-1] = nums2_elem;
                n_left = n_left - 1;
            } else {
                nums1[numbers_left-1] = nums1_elem;
                m_left = m_left - 1;
            }
            numbers_left = numbers_left - 1;
        }
        while m_left > 0 {
            nums1[numbers_left-1] = nums1[m_left-1];
            m_left = m_left - 1;
            numbers_left = numbers_left - 1;
        }
        while n_idx > 0 {
            nums1[numbers_left-1] = nums2[n_left-1];
            n_left = n_left - 1;
            numbers_left = numbers_left - 1;
        }
    }
}

检查 usize 中整数溢出的规范方法

Canonical way to check for integer overflow in usize

rust

unsigned

integer-overflow

unsigned-integer