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c++_c++编程基础之leetcode题解第8题字符串转换整数.zip （1个子文件）

c++_c++编程基础之leetcode题解第8题字符串转换整数

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# [8. String to Integer (atoi) (Medium)](https://leetcode.com/problems/string-to-integer-atoi/) Implement the <code>myAtoi(string s)</code> function, which converts a string to a 32-bit signed integer (similar to C/C++'s <code>atoi</code> function). The algorithm for <code>myAtoi(string s)</code> is as follows: <ol> <li>Read in and ignore any leading whitespace.</li> <li>Check if the next character (if not already at the end of the string) is <code>'-'</code> or <code>'+'</code>. Read this character in if it is either. This determines if the final result is negative or positive respectively. Assume the result is positive if neither is present.</li> <li>Read in next the characters until the next non-digit character or the end of the input is reached. The rest of the string is ignored.</li> <li>Convert these digits into an integer (i.e. <code>"123" -> 123</code>, <code>"0032" -> 32</code>). If no digits were read, then the integer is <code>0</code>. Change the sign as necessary (from step 2).</li> <li>If the integer is out of the 32-bit signed integer range <code>[-231, 231 - 1]</code>, then clamp the integer so that it remains in the range. Specifically, integers less than <code>-231</code> should be clamped to <code>-231</code>, and integers greater than <code>231 - 1</code> should be clamped to <code>231 - 1</code>.</li> <li>Return the integer as the final result.</li> </ol> Note: <ul> <li>Only the space character <code>' '</code> is considered a whitespace character.</li> <li>Do not ignore any characters other than the leading whitespace or the rest of the string after the digits.</li> </ul>   Example 1: <pre>Input: s = "42" Output: 42 Explanation: The underlined characters are what is read in, the caret is the current reader position. Step 1: "42" (no characters read because there is no leading whitespace) ^ Step 2: "42" (no characters read because there is neither a '-' nor '+') ^ Step 3: "42" ("42" is read in) ^ The parsed integer is 42. Since 42 is in the range [-231, 231 - 1], the final result is 42. </pre> Example 2: <pre>Input: s = " -42" Output: -42 Explanation: Step 1: " -42" (leading whitespace is read and ignored) ^ Step 2: " -42" ('-' is read, so the result should be negative) ^ Step 3: " -42" ("42" is read in) ^ The parsed integer is -42. Since -42 is in the range [-231, 231 - 1], the final result is -42. </pre> Example 3: <pre>Input: s = "4193 with words" Output: 4193 Explanation: Step 1: "4193 with words" (no characters read because there is no leading whitespace) ^ Step 2: "4193 with words" (no characters read because there is neither a '-' nor '+') ^ Step 3: "4193 with words" ("4193" is read in; reading stops because the next character is a non-digit) ^ The parsed integer is 4193. Since 4193 is in the range [-231, 231 - 1], the final result is 4193. </pre>   Constraints: <ul> <li><code>0 <= s.length <= 200</code></li> <li><code>s</code> consists of English letters (lower-case and upper-case), digits (<code>0-9</code>), <code>' '</code>, <code>'+'</code>, <code>'-'</code>, and <code>'.'</code>.</li> </ul> **Companies**: [Facebook](https://leetcode.com/company/facebook), [Microsoft](https://leetcode.com/company/microsoft), [Amazon](https://leetcode.com/company/amazon), [Bloomberg](https://leetcode.com/company/bloomberg), [Google](https://leetcode.com/company/google), [Goldman Sachs](https://leetcode.com/company/goldman-sachs), [Adobe](https://leetcode.com/company/adobe), [Qualtrics](https://leetcode.com/company/qualtrics) **Related Topics**: [String](https://leetcode.com/tag/string/) **Similar Questions**: * [Reverse Integer (Medium)](https://leetcode.com/problems/reverse-integer/) * [Valid Number (Hard)](https://leetcode.com/problems/valid-number/) * [Check if Numbers Are Ascending in a Sentence (Easy)](https://leetcode.com/problems/check-if-numbers-are-ascending-in-a-sentence/) ## Solution 1. ```cpp // OJ: https://leetcode.com/problems/string-to-integer-atoi/ // Author: github.com/lzl124631x // Time: O(N) // Space: O(1) class Solution { public: int myAtoi(string s) { int i = 0, N = s.size(), sign = 1, ans = 0; while (i < N && s[i] == ' ') ++i; if (i < N && (s[i] == '+' || s[i] == '-')) sign = s[i++] == '+' ? 1 : -1; while (i < N && isdigit(s[i])) { int n = s[i++] - '0'; if (ans > INT_MAX / 10 || (ans == INT_MAX / 10 && n > INT_MAX % 10)) return sign == 1 ? INT_MAX : INT_MIN; ans = ans * 10 + n; } return ans * sign; } }; ```

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