1663. Smallest String With A Given Numeric Value

Question

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 <= 105 n <= k <= 26 * n

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Approach

1. Create a string of len n (the length of return string) and populate it with 'a' (the smallest in lexicographically order).
2. k is the total max, k - n is the max string needed.
3. Check k and 25 (= z), whichever that is smaller will be placed at the back.
4. Minus the string number of the places char, and continue until k is 0.
5. The rest of the string will be kept as a.

Solution

class Solution {
public:
    string getSmallestString(int n, int k) {
        string res(n, 'a');
        k -= n;
        
        int i = res.size() - 1;
        while (k > 0) {
            int tmp = min(k, 25);
            res[i] += tmp;
            k -= tmp;
            --i;
        }
        
        return res;
    }
};