The numeric value of a lowercase character is defined as its position (1-indexed) in the alphabet, so the numeric value of a is 1, the numeric value of b is 2, the numeric value of c is 3, and so on.

The numeric value of a string consisting of lowercase characters is defined as the sum of its characters’ numeric values. For example, the numeric value of the string "abe" is equal to 1 + 2 + 5 = 8.

You are given two integers n and k. Return the lexicographically smallest string with length equal to n and numeric value equal to k.

Note that a string x is lexicographically smaller than string y if x comes before y in dictionary order, that is, either x is a prefix of y, or if i is the first position such that x[i] != y[i], then x[i] comes before y[i] in alphabetic order.

Example 1:

Input: n = 3, k = 27
Output: "aay"
Explanation: The numeric value of the string is 1 + 1 + 25 = 27, and it is the smallest string with such a value and length equal to 3.

Example 2:

Input: n = 5, k = 73
Output: "aaszz"

Constraints:

1 <= n <= 10^5
n <= k <= 26 * n

Solution

Time complexity : O(n)
Space complexity : O(n)

class Solution {
public:
    string getSmallestString(int n, int k) {
        string ans(n, 'a');
        k -= n;
        while (0 < k) {
            int diff = min(k, 25);
            ans[--n] += diff;
            k -= diff;
        }
        return ans;
    }
};

先用 a 鋪地毯，再由後往前加至最大的字母。