## [290\. Word Pattern](https://leetcode.com/problems/word-pattern/)

Given a `pattern` and a string `s`, find if `s` follows the same pattern.

Here **follow** means a full match, such that there is a bijection between a letter in `pattern` and a **non-empty** word in `s`.

**Example 1:**

**Input:** pattern = "abba", s = "dog cat cat dog"
**Output:** true

**Example 2:**

**Input:** pattern = "abba", s = "dog cat cat fish"
**Output:** false

**Example 3:**

**Input:** pattern = "aaaa", s = "dog cat cat dog"
**Output:** false

**Constraints:**

- `1 <= pattern.length <= 300`
- `pattern` contains only lower-case English letters.
- `1 <= s.length <= 3000`
- `s` contains only lowercase English letters and spaces `' '`.
- `s` **does not contain** any leading or trailing spaces.
- All the words in `s` are separated by a **single space**.

class Solution {
public:
    bool wordPattern(string pattern, string s) {
        unordered_map<char, int> m1;
        unordered_map<string, int> m2;

        stringstream ss(s);
        int i = 0;
        int n = pattern.size();
        for (string word; ss >> word; ++i) {
            // 如果遇到 i == n 的情況，代表 pattern 已經先到終點了
            // 但 word 還有單字
            if (i == n || m1[pattern[i]] != m2[word]) {
                return false;
            }
            // 更新 index + 1
            m1[pattern[i]] = i + 1;
            m2[word] = i + 1;
        }
        // 最後比較 i 有沒有到達終點
        return i == n;
    }
};

:::success
- 時間複雜度：$O(n + m)$
- 空間複雜度：$O(n)$
:::