278. First Bad Version - LeetCode

Question

278. First Bad Version java

Solution

題目大意:產品有5個版本1,2,3,4,5其中下一個版本依賴上一個版本,即版本4是壞的,5也就是壞的,如今要求哪一個版本是第一個壞的。code

思路:二分法,middle-1好,middle壞,middle就是第一個壞的版本ip

Java實現:leetcode

/* 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;
    }
}

參考:get

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;
}
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