LeetCode-探索-初級算法-排序和搜索-2. 第一個錯誤的版本(個人做題記錄，不是習題講解)

LeetCode-探索-初級算法-排序和搜索-2. 第一個錯誤的版本(個人做題記錄，不是習題講解)

LeetCode探索-初級算法：https://leetcode-cn.com/explore/interview/card/top-interview-questions-easy/

2. 第一個錯誤的版本

• 語言：java

• 思路：二分查找，每次取中間和中間的右邊那個進行判斷。

• 代碼(19ms,擊敗23.74%，丟人，哈哈)：

  /* The isBadVersion API is defined in the parent class VersionControl.
        boolean isBadVersion(int version); */
  
  public class Solution extends VersionControl {
      public int firstBadVersion(int n) {
          if(n>1){
              int start = 1;
              int end = n; 
              int middle;
              boolean left = false;
              boolean right = false;
              while(start<=end){
                  middle = start + (end-start)/2;
                  left = isBadVersion(middle);
                  if(!left){
                      right = isBadVersion(middle+1);
                      if(right)
                          return middle+1;
                      else{
                          start = middle+1;
                      }
                  }
                  else{
                      end = middle - 1;
                  }
              }
          }
          return 1;
      }
  }
  

• 參考代碼(11ms)：其實思想都是二分查找，就是我的實現代碼太low了。

  public class Solution extends VersionControl {
      public int firstBadVersion(int n) {
  
          int l = 1, r = n;
          while (l < r) {
              int mid = (l + r) >>> 1;
              if (isBadVersion(mid)) {
                  r = mid;
                  // System.out.println("badVersion: " + mid);
              } else {
                  // System.out.println("goodVersion: " + mid);
                  l = mid + 1;
              }
  
              // System.out.println("l :" + l + " r:" + r);
          }
          return r;
      }
  }
  

• 參考後重寫(12ms)：主要是要清楚最後返回的是end，因爲end就是符合條件的那個

  /* 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 start = 1;
          int end = n;
          int middle;
          while(start<end){
              middle = (start+end)>>>1;
              if(isBadVersion(middle)){
                  end = middle;
              }else{
                  start = middle + 1;
              }
          }
          return end;
      }
  }