<p>You are given a string <code>word</code> and an array of strings <code>forbidden</code>.</p>

<p>A string is called <strong>valid</strong> if none of its substrings are present in <code>forbidden</code>.</p>

<p>Return <em>the length of the <strong>longest valid substring</strong> of the string </em><code>word</code>.</p>

<p>A <strong>substring</strong> is a contiguous sequence of characters in a string, possibly empty.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> word = &quot;cbaaaabc&quot;, forbidden = [&quot;aaa&quot;,&quot;cb&quot;]
<strong>Output:</strong> 4
<strong>Explanation:</strong> There are 11 valid substrings in word: &quot;c&quot;, &quot;b&quot;, &quot;a&quot;, &quot;ba&quot;, &quot;aa&quot;, &quot;bc&quot;, &quot;baa&quot;, &quot;aab&quot;, &quot;ab&quot;, &quot;abc&quot; and &quot;aabc&quot;. The length of the longest valid substring is 4. 
It can be shown that all other substrings contain either &quot;aaa&quot; or &quot;cb&quot; as a substring. </pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> word = &quot;leetcode&quot;, forbidden = [&quot;de&quot;,&quot;le&quot;,&quot;e&quot;]
<strong>Output:</strong> 4
<strong>Explanation:</strong> There are 11 valid substrings in word: &quot;l&quot;, &quot;t&quot;, &quot;c&quot;, &quot;o&quot;, &quot;d&quot;, &quot;tc&quot;, &quot;co&quot;, &quot;od&quot;, &quot;tco&quot;, &quot;cod&quot;, and &quot;tcod&quot;. The length of the longest valid substring is 4.
It can be shown that all other substrings contain either &quot;de&quot;, &quot;le&quot;, or &quot;e&quot; as a substring. 
</pre>

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

<ul>
	<li><code>1 &lt;= word.length &lt;= 10<sup>5</sup></code></li>
	<li><code>word</code> consists only of lowercase English letters.</li>
	<li><code>1 &lt;= forbidden.length &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= forbidden[i].length &lt;= 10</code></li>
	<li><code>forbidden[i]</code> consists only of lowercase English letters.</li>
</ul>

class Solution:
    def longestValidSubstring(self, word: str, forbidden: List[str]) -> int:
        # construct trie for forbidden words
        trie = {}
        for f_word in forbidden:
            cur_node = trie
            for char in f_word:
                if char not in cur_node:
                    cur_node[char] = {}
                cur_node = cur_node[char]
            cur_node['end'] = {}

        print(trie)

        def is_forbidden(s):
            node = trie
            for i,char in enumerate(s):
                if char in node:
                    node = node[char]
                else:
                    return False
                if 'end' in node:
                    return i+1
                
        _max = 0
        r = len(word)
        for l in range(len(word)-1,-1,-1):
            temp = word[l:r]
            idx = is_forbidden(temp)
            if idx:
                r = l+idx-1
            _max = max(_max,r-l)

        return _max

# summary:
# 1. Use trie to represent forbidden words so that search can be done faster
# 2. Move the left pointer from "end of word" to "start of word". 
#    This is because we want to check if the newly added character forms forbidden word with other characters
#    and we check the substring from left to right
# 3. To avoid '0' confuses with 'False' in the if statement, we use 'i+1'

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