Question

988. Smallest String Starting From Leaf

中文文档

Description

You are given the root of a binary tree where each node has a value in the range [0, 25] representing the letters 'a' to 'z'.

Return _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: root = [0,1,2,3,4,3,4]
Output: "dba"

Example 2:

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

Example 3:

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

 

Constraints:

• The number of nodes in the tree is in the range [1, 8500].
• 0 <= Node.val <= 25

Solutions

Solution 1

# Definition for a binary tree node.
# class TreeNode:
#   def __init__(self, val=0, left=None, right=None):
#     self.val = val
#     self.left = left
#     self.right = right
class Solution:
  def smallestFromLeaf(self, root: TreeNode) -> str:
    ans = chr(ord('z') + 1)

    def dfs(root, path):
      nonlocal ans
      if root:
        path.append(chr(ord('a') + root.val))
        if root.left is None and root.right is None:
          ans = min(ans, ''.join(reversed(path)))
        dfs(root.left, path)
        dfs(root.right, path)
        path.pop()

    dfs(root, [])
    return ans

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *   int val;
 *   TreeNode left;
 *   TreeNode right;
 *   TreeNode() {}
 *   TreeNode(int val) { this.val = val; }
 *   TreeNode(int val, TreeNode left, TreeNode right) {
 *     this.val = val;
 *     this.left = left;
 *     this.right = right;
 *   }
 * }
 */
class Solution {
  private StringBuilder path;
  private String ans;

  public String smallestFromLeaf(TreeNode root) {
    path = new StringBuilder();
    ans = String.valueOf((char) ('z' + 1));
    dfs(root, path);
    return ans;
  }

  private void dfs(TreeNode root, StringBuilder path) {
    if (root != null) {
      path.append((char) ('a' + root.val));
      if (root.left == null && root.right == null) {
        String t = path.reverse().toString();
        if (t.compareTo(ans) < 0) {
          ans = t;
        }
        path.reverse();
      }
      dfs(root.left, path);
      dfs(root.right, path);
      path.deleteCharAt(path.length() - 1);
    }
  }
}

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *   int val;
 *   TreeNode *left;
 *   TreeNode *right;
 *   TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *   TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *   TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
  string ans = "";

  string smallestFromLeaf(TreeNode* root) {
    string path = "";
    dfs(root, path);
    return ans;
  }

  void dfs(TreeNode* root, string& path) {
    if (!root) return;
    path += 'a' + root->val;
    if (!root->left && !root->right) {
      string t = path;
      reverse(t.begin(), t.end());
      if (ans == "" || t < ans) ans = t;
    }
    dfs(root->left, path);
    dfs(root->right, path);
    path.pop_back();
  }
};

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *   Val int
 *   Left *TreeNode
 *   Right *TreeNode
 * }
 */
func smallestFromLeaf(root *TreeNode) string {
  ans := ""
  var dfs func(root *TreeNode, path string)
  dfs = func(root *TreeNode, path string) {
    if root == nil {
      return
    }
    path = string('a'+root.Val) + path
    if root.Left == nil && root.Right == nil {
      if ans == "" || path < ans {
        ans = path
      }
      return
    }
    dfs(root.Left, path)
    dfs(root.Right, path)
  }

  dfs(root, "")
  return ans
}