smallest-string-starting-from-leaf.md

<p>Given the <code>root</code> of a binary tree, each node has a value from <code>0</code> to <code>25</code> representing the letters <code>&#39;a&#39;</code> to <code>&#39;z&#39;</code>: a value of <code>0</code> represents <code>&#39;a&#39;</code>, a value of <code>1</code> represents <code>&#39;b&#39;</code>, and so on.</p>

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

<p><em>(As a reminder, any shorter prefix of a string is lexicographically smaller: for example, <code>&quot;ab&quot;</code> is lexicographically smaller than <code>&quot;aba&quot;</code>.&nbsp; A leaf of a node is a node that has no children.)</em></p>

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<p><strong>Example 1:</strong></p>

<p><strong><img alt="" src="https://assets.leetcode.com/uploads/2019/01/30/tree1.png" style="width: 160px; height: 107px;" /></strong></p>

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<strong>Input: </strong><span id="example-input-1-1">[0,1,2,3,4,3,4]</span>
<strong>Output: </strong><span id="example-output-1">&quot;dba&quot;</span>
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<p><strong>Example 2:</strong></p>

<p><strong><img alt="" src="https://assets.leetcode.com/uploads/2019/01/30/tree2.png" style="width: 160px; height: 107px;" /></strong></p>

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<strong>Input: </strong><span id="example-input-2-1">[25,1,3,1,3,0,2]</span>
<strong>Output: </strong><span id="example-output-2">&quot;adz&quot;</span>
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<p><strong>Example 3:</strong></p>

<p><strong><img alt="" src="https://assets.leetcode.com/uploads/2019/02/01/tree3.png" style="height: 170px; width: 172px;" /></strong></p>

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<strong>Input: </strong><span id="example-input-3-1">[2,2,1,null,1,0,null,0]</span>
<strong>Output: </strong><span id="example-output-3">&quot;abc&quot;</span>
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<p><strong>Note:</strong></p>

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	<li>The number of nodes in the given tree will be between <code>1</code> and <code>8500</code>.</li>
	<li>Each node in the tree will have a value between <code>0</code> and <code>25</code>.</li>
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