[leetcode]LeetCode刷题的日子--integer to roman

题目

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XII, which is simply X + II. The number twenty seven is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

• I can be placed before V (5) and X (10) to make 4 and 9.
• X can be placed before L (50) and C (100) to make 40 and 90.
• C can be placed before D (500) and M (1000) to make 400 and 900.

Given an integer, convert it to a roman numeral. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

Input: 3
Output: "III"

Example 2:

Input: 4
Output: "IV"

Example 3:

Input: 9
Output: "IX"

Example 4:

Input: 58
Output: "LVIII"
Explanation: L = 50, V = 5, III = 3.

Example 5:

Input: 1994
Output: "MCMXCIV"
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

解题思路

解题的基本思路，就是构建一个stack，最上层为1000，然后依次为900, 500, 400, 100, 90, 50, 40, 10, 5, 4, 1, 然后将需要进行转换的数字，依次与最上层的数据进行比对，如果大于该值，则积累一个该层对应的字符，然后减掉该层数字。

如果小于该层数字，则从stack中pop掉该层，重复上述步骤，直到stack为空

代码

fn int_to_roman(num: i32) -> String {
    let mut num = num;
    let mut table = vec![(1, "I"),
        (4, "IV"),
        (5, "V"),
        (9, "IX"),
        (10, "X"),
        (40, "XL"),
        (50, "L"),
        (90, "XC"),
        (100, "C"),
        (400, "CD"),
        (500, "D"),
        (900, "CM"),
        (1000, "M")
    ];

    let mut ret = String::from("");

    while let Some(&(value, roman)) = table.last() {
        if num < value {
            table.pop();
        } else {
            num -= value;
            ret += roman;
        }
    }

    return ret;
}

#[test]
fn test_int_to_roman() {
    assert_eq!(int_to_roman(3), "III");
    assert_eq!(int_to_roman(4), "IV");
    assert_eq!(int_to_roman(9), "IX");
    assert_eq!(int_to_roman(58), "LVIII");
    assert_eq!(int_to_roman(1994), "MCMXCIV")
}