278. First Bad Version - LeetCode

Question

278. First Bad Version

Solution

题目大意：产品有5个版本1，2，3，4，5其中下一个版本依赖上一个版本，即版本4是坏的，5也就是坏的，现在要求哪个版本是第一个坏的。

思路：二分法，middle-1好，middle坏，middle就是第一个坏的版本

Java实现：

/* The isBadVersion API is defined in the parent class VersionControl.
      boolean isBadVersion(int version); */

public class Solution extends VersionControl {

    public int firstBadVersion(int n) {
        int left = 1, right = n;
        int badVersion = -1;
        while (badVersion == -1) {
            int middle = left + (right-left)/2;
            boolean pre = isBadVersion(middle-1);
            boolean cur = isBadVersion(middle);
            if (!pre && cur) {
                badVersion = middle;
            } else {
                // false true     middle is BadVersion
                // false false    left = middle + 1;
                // true false     x
                // true true      right = middle - 1;
                if (pre) {
                    right = middle-1;
                } else {
                    left = middle + 1;
                }
            }
        }
        return badVersion;
    }
}

参考：

public int firstBadVersion(int n) {
    int start = 1, end = n;
    while (start < end) {
        int mid = start + (end-start) / 2;
        if (!isBadVersion(mid)) start = mid + 1;
        else end = mid;
    }
    return start;
}