問題：請實現以下函式int indexOf(int [] array ,int key) ,給定一個迴圈有序的陣列，請在這個陣列中找到指定元素，找到的話返回下標，沒找到返回-1。：

解決：首先，使用二分查詢找到陣列的 “臨界點”,臨界點滿足兩個情況：
1. array[left] < array[mid]
2. array[left] <= array[mid]
只有確定分界點，確定了key的範圍，才能找到key的位置.

程式碼塊

public class BinaryFindCycleArr {
        public
 
 static  int FindCycleArr(int[] arr,int key){
        int left = 0;
        int right = arr.length-1;
        while (left <= right) {
            int mid = (left + right)/2;
            if (key == arr[mid]) {
                return mid;
            }
            if (arr[left] <= arr[mid]) {
                if
 
 (key < arr[mid] && key >= arr[left]) {
                    right = mid-1;
                 }else {
                    left = mid+1;
                }
            }else if (arr[left] > arr[mid]) {
                if (key < arr[left] && key > arr[mid]) {
                    left
 
 = mid + 1;
                }else {
                    right= mid - 1;
                }
            }
        }
        return -1;
    }
    public static void main(String[] args) {
        int[] arr = new int[]{23,34,41,52,4,8,13,20};
        for (int i = 0; i < arr.length; i++) {
            int out =FindCycleArr(arr,arr[i]);
            System.out.println(out);
        }
}

問題延伸：如果限制只找到最大值，程式碼如下：

public static  int FindMax(int[] arr){
    int left = 0;
    int right = arr.length-1;
    int max = arr[0];
    while (left <= right) {
        int mid = (left + right)/2;
        if (max >arr[mid]) {
            right = right-1;
        }else {
            max=arr[mid];
            left = mid+1;
        }
    }
    return max;
}

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