<p>We are given <code>n</code> different types of <code>stickers</code>. Each sticker has a lowercase English word on it.</p>

<p>You would like to spell out the given string <code>target</code> by cutting individual letters from your collection of stickers and rearranging them. You can use each sticker more than once if you want, and you have infinite quantities of each sticker.</p>

<p>Return <em>the minimum number of stickers that you need to spell out </em><code>target</code>. If the task is impossible, return <code>-1</code>.</p>

<p><strong>Note:</strong> In all test cases, all words were chosen randomly from the <code>1000</code> most common US English words, and <code>target</code> was chosen as a concatenation of two random words.</p>

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

<pre>
<strong>Input:</strong> stickers = [&quot;with&quot;,&quot;example&quot;,&quot;science&quot;], target = &quot;thehat&quot;
<strong>Output:</strong> 3
<strong>Explanation:</strong>
We can use 2 &quot;with&quot; stickers, and 1 &quot;example&quot; sticker.
After cutting and rearrange the letters of those stickers, we can form the target &quot;thehat&quot;.
Also, this is the minimum number of stickers necessary to form the target string.
</pre>

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

<pre>
<strong>Input:</strong> stickers = [&quot;notice&quot;,&quot;possible&quot;], target = &quot;basicbasic&quot;
<strong>Output:</strong> -1
Explanation:
We cannot form the target &quot;basicbasic&quot; from cutting letters from the given stickers.
</pre>

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

<ul>
	<li><code>n == stickers.length</code></li>
	<li><code>1 &lt;= n &lt;= 50</code></li>
	<li><code>1 &lt;= stickers[i].length &lt;= 10</code></li>
	<li><code>1 &lt;= target.length &lt;= 15</code></li>
	<li><code>stickers[i]</code> and <code>target</code> consist of lowercase English letters.</li>
</ul>

from collections import Counter

INF = float("inf")
class Solution:
    def minStickers(self, stickers: List[str], target: str) -> int:
        # 5:16 - 5:31
        fmap = Counter(target)
        self.stickers = stickers
        self.n = len(stickers)
        # backtracking
        # memoization
        self.cache = dict()
        ans = self.backtrack(0, fmap)
        if ans == INF:
            return -1
        return ans
        
    def backtrack(self, idx: int, rest: dict) -> int:
        if not rest:
            return 0
        # stopping condition
        if idx == self.n:
            return INF

        key = (idx, str(rest))
        if key in self.cache:
            return self.cache[key]

        ans = INF
        # check if this sticker has any use
        # is useful if has any letter in rest
        # try using this sticker
        after_used = dict(rest)
        is_useful = False
        for c in self.stickers[idx]:
            if c in after_used:
                after_used[c] -= 1
                if after_used[c] == 0:
                    del after_used[c]
                is_useful = True
        if is_useful:
            # go to the next backtrack with this sticker
            ans = 1 + self.backtrack(idx, after_used)

        # go to the next sticker directly
        ans = min(ans, self.backtrack(idx+1, rest))
        self.cache[key] = ans
        return ans

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