String to Integer (atoi)

Author: w2xi

Diff for: README.md

@@ -5,6 +5,7 @@
 |1|[两数之和](https://leetcode-cn.com/problems/two-sum/)|[JavaScript](./algorithms/two-sum.js)|Easy|
 |2|[两数相加](https://leetcode-cn.com/problems/add-two-numbers/)|[JavaScript](./algorithms/add-two-numbers.js)|Medium|
 |7|[整数反转](https://leetcode-cn.com/problems/reverse-integer/)|[JavaScript](./algorithms/reverse-integer.js)|Medium|
+|8|[字符串转换整数 (atoi)](https://leetcode-cn.com/problems/string-to-integer-atoi/)|[JavaScript](./algorithms/string-to-integer-atoi.js)|Medium|
 |9|[回文数](https://leetcode-cn.com/problems/palindrome-number/)|[JavaScript](./algorithms/palindrome-number.js)|Easy|
 |14|[最长公共前缀](https://leetcode-cn.com/problems/longest-common-prefix/)|[JavaScript](./algorithms/longest-common-prefix.js)|Easy|
 |21|[合并两个有序链表](https://leetcode-cn.com/problems/merge-two-sorted-lists/)|[JavaScript](./algorithms/merge-two-sorted-lists.js)|Easy|

Diff for: algorithms/string-to-integer-atoi.js

@@ -0,0 +1,103 @@
+// 方法一：一般解法
+
+/**
+ * 字符串转换整数 (atoi)
+ * @param {string} s
+ * @return {number}
+ */
+var myAtoi = function (s) {
+  let flag = 1; // 1:正数 1-:负数
+
+  s = s.trimStart();
+
+  if (s[0] === "-") {
+    flag = -1;
+    s = s.slice(1);
+  } else if (s[0] === "+") {
+    s = s.slice(1);
+  }
+
+  let arr = [];
+  let count = 0;
+
+  for (let i = 0; i < s.length; i++) {
+    let curr = s[i];
+
+    if (+curr === +curr && count === i && curr !== " ") {
+      arr.push(curr);
+      count++;
+    }
+  }
+
+  let result = arr.length > 0 ? Number(arr.join("")) : 0;
+  result *= flag;
+
+  const min = -Math.pow(2, 31);
+  const max = Math.pow(2, 31) - 1;
+
+  if (result < min) {
+    result = min;
+  } else if (result > max) result = max;
+
+  return result;
+};
+
+// 方法二：有限自动机 (DFA)
+
+/**
+ * 字符串转换整数 (atoi)
+ * @param {string} s
+ * @return {number}
+ */
+var myAtoi = function (s) {
+  const am = automaton();
+
+  for (let i = 0; i < s.length; i++) {
+    am.get(s[i]);
+  }
+  
+  return am.result()
+};
+
+const MAX_NUMBER = Math.pow(2, 31) - 1;
+const MIN_NUMBER = -Math.pow(2, 31);
+
+function automaton() {
+  let sign = 1, ans = 0, state = "start";
+  let table = {
+    start: ["start", "signed", "in_number", "end"],
+    signed: ["end", "end", "in_number", "end"],
+    in_number: ["end", "end", "in_number", "end"],
+    end: ["end", "end", "end", "end"],
+  };
+
+  return {
+    get(c) {
+      state = table[state][this.getCol(c)];
+
+      if (state === "in_number") {
+        ans = ans * 10 + Number(c);
+        ans = sign === 1 ? Math.min(ans, MAX_NUMBER) : Math.min(ans, -MIN_NUMBER);
+      } else if (state === "signed") {
+        sign = c === "-" ? -1 : 1;
+      }
+    },
+
+    getCol(c) {
+      if (c == " ") {
+        return 0;
+      }
+      if (c == "+" || c == "-") {
+        return 1;
+      }
+      if (/\d/.test(c)) {
+        return 2;
+      }
+      return 3;
+    },
+
+    result() {
+      return sign * ans;
+    },
+  };
+}