Smallest String Starting From Leaf

Description

Given the root of a binary tree, each node has a value from 0 to 25 representing the letters 'a' to 'z': a value of 0 represents 'a', a value of 1 represents 'b', and so on.

Find the lexicographically smallest string that starts at a leaf of this tree and ends at the root.

(As a reminder, any shorter prefix of a string is lexicographically smaller: for example, "ab" is lexicographically smaller than "aba".  A leaf of a node is a node that has no children.)

 

Example 1:

Input: [0,1,2,3,4,3,4]
Output: "dba"

Example 2:

Input: [25,1,3,1,3,0,2]
Output: "adz"

Example 3:

Input: [2,2,1,null,1,0,null,0]
Output: "abc"

 

Note:

1. The number of nodes in the given tree will be between 1 and 8500.
2. Each node in the tree will have a value between 0 and 25.

Solution(javascript)

const smallestFromLeaf = function (root) {
  let min = null
  const aux = (node = root, current = '') => {
    const str = String.fromCharCode(97 + node.val) + current
    if (!node.left && !node.right) {
      if (!min) {
        min = str
      } else {
        min = str.localeCompare(min) < 0 ? str : min
      }
    }
    if (node.left) {
      aux(node.left, str)
    }
    if (node.right) {
      aux(node.right, str)
    }
  }
  aux()
  return min
}