387. First Unique Character in a String

387/memo.md

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+# 387. First Unique Character in a String
+
+- 愚直な実装（sol1.py）は2回文字を見る
+- https://discord.com/channels/1084280443945353267/1233603535862628432/1238208008182562927
+    - 文字が一回しか流れてこない場合を考えたい(sol2.py)
+    - Ordered dictを使うと書ける
+    - 速くなった
+
+- Queueも使う実装もある
+    - https://github.com/colorbox/leetcode/pull/29/changes/BASE..48f2749be9c4ec78c6f24c887880e34c7206f678#r1861430039
+    - sol3.py
+    - queue.Queueにはpeek（先頭の覗き見）がない
+    - queueメソッドを使ったが推奨されないと思う
+    - dequeを使うのが良い気がする
+- OrderedDictの実装はdoubly-linked list
+    - 実装した (sol4.py)
+    - 参考：https://gist.github.com/joequery/12332f410a05e6c7c949
+    - "Doubly"によって前の参照ができることにより、削除をO(1)で実行可能
+    - 「これ、OrderedDict の中身は Doubly-Linked List なので、まあ、練習としては、Doubly-Linked List 自体を書いて欲しいところではありますね。」

387/sol1.py

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+START_CHAR = "a"
+START_CHAR_ORD = ord(START_CHAR)
+NUM_CHARS = 26
+
+
+class Solution:
+    def firstUniqChar(self, s: str) -> int:
+        ord_to_count = [0] * NUM_CHARS
+        for c in s:
+            ord_to_count[ord(c) - START_CHAR_ORD] += 1
+        for i, c in enumerate(s):
+            if ord_to_count[ord(c) - START_CHAR_ORD] == 1:
+                return i
+        return -1

387/sol2.py

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+from collections import OrderedDict
+
+
+class Solution:
+    def firstUniqChar(self, s: str) -> int:
+        seen_chars = set()
+        char_to_idx = OrderedDict()
+        for i, c in enumerate(s):
+            if c in seen_chars:
+                if c in char_to_idx:
+                    del char_to_idx[c]
+                continue
+            seen_chars.add(c)
+            char_to_idx[c] = i
+        if char_to_idx:
+            return next(iter(char_to_idx.values()))
+        return -1

387/sol3.py

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+import queue
+
+
+class Solution:
+    def firstUniqChar(self, s: str) -> int:
+        import queue
+        from collections import defaultdict
+
+        counts = defaultdict(int)
+        count_and_idxs = queue.Queue()
+
+        for i, c in enumerate(s):
+            counts[c] += 1
+            count_and_idxs.put((c, i))
+
+            while not count_and_idxs.empty():
+                front_c, _front_i = count_and_idxs.queue[0]
+                if counts[front_c] == 1:
+                    break
+                count_and_idxs.get()
+
+        return count_and_idxs.queue[0][1] if not count_and_idxs.empty() else -1

387/sol4.py

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+class Link:
+    def __init__(self, prev=None, next=None, key=None):
+        self.prev = prev
+        self.next = next
+        self.key = key
+
+
+class CustomOrderedDict(dict):
+    def __init__(self):
+        super().__init__()
+        self.root = Link()
+        self.root.prev = self.root.next = self.root
+        self.link_map = {}
+
+    def __getitem__(self, key):
+        return super().__getitem__(key)
+
+    def __setitem__(self, key, value):
+        if key not in self:
+            last = self.root.prev
+            new_link = Link(prev=last, next=self.root, key=key)
+            last.next = self.root.prev = new_link
+            self.link_map[key] = new_link
+        super().__setitem__(key, value)
+
+    def __delitem__(self, key):
+        super().__delitem__(key)
+        link = self.link_map.pop(key)
+        link.prev.next = link.next
+        link.next.prev = link.prev
+
+    def __iter__(self):
+        curr = self.root.next
+        while curr is not self.root:
+            yield curr.key
+            curr = curr.next
+
+
+class Solution:
+    def firstUniqChar(self, s: str) -> int:
+        seen_chars = set()
+        char_to_idx = CustomOrderedDict()
+        for i, c in enumerate(s):
+            if c in seen_chars:
+                if c in char_to_idx:
+                    del char_to_idx[c]
+                continue
+            seen_chars.add(c)
+            char_to_idx[c] = i
+        if char_to_idx:
+            return next(iter(char_to_idx.values()))
+        return -1