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+设 substr 的起点为 start(s), 终点为 last(l). 每一次迭代，记录一张索引表。
+
+    abcabcbb
+    ^  ^
+    s  l
+    
+|char|pos|
+|:--:|:-:|
+| a  | 0 |
+| b  | 1 |
+| c  | 2 |
+
+上图所示，last 指向 `a`, 查询当前表可知，`a` 的位置记录在案，且 `pos >= start`. 故此刻诞生一个 substr. 长度为 `last - start`. s 更新位置为 `pos + 1`.
+
+有：
+
+```cpp
+auto found = cache.find(s[last]);
+if (found != cache.end() && found->second >= start) {
+    ret = max(ret, last - start);
+    start = found->second + 1;
+}
+cache[s[last]] = last;
+```
+
+注意最终还需要比较一次，返回 `max(ret, s.size() - start)`
+
+
+
+
+