28. Smallest String With A Given Numeric Value.cpp

/*
Smallest String With A Given Numeric Value
===========================================

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

Hint #1  
Think greedily.

Hint #2  
If you build the string from the end to the beginning, it will always be optimal to put the highest possible character at the current index.
*/

class Solution
{
public:
  string getSmallestString(int n, int k)
  {
    string ans(n, 'a');
    int i = n - 1, count = n;
    while (count < k)
    {
      if (ans[i] == 'z')
        i--;
      else
      {
        ans[i]++;
        count++;
      }
    }
    return ans;
  }
};