<p>You are given the <code>root</code> of a binary tree where each node has a value in the range <code>[0, 25]</code> representing the letters <code>&#39;a&#39;</code> to <code>&#39;z&#39;</code>.</p>

<p>Return <em>the <strong>lexicographically smallest</strong> string that starts at a leaf of this tree and ends at the root</em>.</p>

<p>As a reminder, any shorter prefix of a string is <strong>lexicographically smaller</strong>.</p>

<ul>
	<li>For example, <code>&quot;ab&quot;</code> is lexicographically smaller than <code>&quot;aba&quot;</code>.</li>
</ul>

<p>A leaf of a node is a node that has no children.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/01/30/tree1.png" style="width: 534px; height: 358px;" />
<pre>
<strong>Input:</strong> root = [0,1,2,3,4,3,4]
<strong>Output:</strong> &quot;dba&quot;
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/01/30/tree2.png" style="width: 534px; height: 358px;" />
<pre>
<strong>Input:</strong> root = [25,1,3,1,3,0,2]
<strong>Output:</strong> &quot;adz&quot;
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/02/01/tree3.png" style="height: 490px; width: 468px;" />
<pre>
<strong>Input:</strong> root = [2,2,1,null,1,0,null,0]
<strong>Output:</strong> &quot;abc&quot;
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 8500]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 25</code></li>
</ul>


class Solution {
public:
    string BFS(TreeNode* root)
    {
        if(!root) return "";
        queue<pair<TreeNode*,string>> q;
        vector<string> ans;
        string s = "";
        s+=root->val+97; 
        q.push({root,s});
        while(!q.empty())
        {
            TreeNode* node = q.front().first;
            string str = q.front().second;
            q.pop();
            if(!node->left && !node->right)
            {
                reverse(str.begin(),str.end());
                ans.push_back(str);
            }
            if(node->left)
            {
                string tmp = str;
                tmp+=node->left->val+97;
                q.push({node->left,tmp});
            }
            if(node->right)
            {
                string tmp = str;
                tmp+=node->right->val+97;
                q.push({node->right,tmp});
            }
        }
        sort(ans.begin(),ans.end());
        return ans[0];
    }
    string smallestFromLeaf(TreeNode* root) {
       return BFS(root);
    }
};

23500