## [解題] LeetCode 392. Is Subsequence Given two strings `s` and `t`, return `true` if `s` is a subsequence of `t`, or `false` otherwise.

subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., `"ace"` is a subsequence of `"abcde"` while `"aec"` is not).

Example 1:

```Input: s = "abc", t = "ahbgdc"
Output: true
```

Example 2:

```Input: s = "axc", t = "ahbgdc"
Output: false
```

Constraints:

• `0 <= s.length <= 100`
• `0 <= t.length <= 104`
• `s` and `t` consist only of lowercase English letters.

Follow up: Suppose there are lots of incoming `s`, say `s1, s2, ..., sk` where `k >= 109`, and you want to check one by one to see if `t` has its subsequence. In this scenario, how would you change your code?

```/**
* @param {string} s
* @param {string} t
* @return {boolean}
*/
var isSubsequence = function(s, t) {
let current = -1
for (let i = 0; i < s.length; i++) {
const idx = t.indexOf(s[i], current + 1)
if (idx > current) {
current = idx
} else {
return false
}
}
return true
};```

### 思路

indexOf 的第二個變數常被忽略：

str.indexOf(searchValue[, fromIndex])

```let str = "Hello World!";

console.log(str.indexOf("o")); // 輸出 4
console.log(str.indexOf("l", 3)); // 輸出 2
console.log(str.indexOf("x")); // 輸出 -1```

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