Leetcode 278. First Bad Version

题目大意：给你一个API查询版本是否坏了，让你找出第一个坏掉的版本。

You are a product manager and currently leading a team to develop a new product. Unfortunately, the latest version of your product fails the quality check. Since each version is developed based on the previous version, all the versions after a bad version are also bad.

Suppose you have n versions [1, 2, ..., n] and you want to find out the first bad one, which causes all the following ones to be bad.

You are given an API bool isBadVersion(version) which will return whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.

Example:

Given n = 5, and version = 4 is the first bad version. call isBadVersion(3) -> false call isBadVersion(5) -> true call isBadVersion(4) -> true Then 4 is the first bad version.

Difficulty:Easy

Category:Binary-Search

Solution

Solution 1: Brute Force

Time Complexity: O(n) TLE Space Complexity: O(1)

// Runtime: 1864 ms, faster than 9.83% of C++ online submissions for First Bad Version.
// Memory Usage: 8.2 MB, less than 41.34% of C++ online submissions for First Bad Version.
bool isBadVersion(int version);

class Solution {
 public:
  int firstBadVersion(int n) {
    for (int i = n; i > 0; --i) {
      if (isBadVersion(i)) continue;
      return i + 1 > n ? n : i + 1;
    }v
    return 1;
  }
};

Solution 2: Binary Search

Time Complexity: O(log n) Space Complexity: O(1)

// Forward declaration of isBadVersion API.
bool isBadVersion(int version);

class Solution {
 public:
  int firstBadVersion(int n) {
    int left = 1, right = n;
    while (left < right) {
      int mid = left + (right - left) / 2;
      isBadVersion(mid) ? right = mid : left = mid + 1;
    }
    return left;
  }
};