求解非递减数组范围下标

Posted 2021-09-03 jeremylai7

题目

有序非递减数组，找出在指定区间中的元素位置，输出起始和结束位置的下标。
如数组： 1,2,2,3,4,6
区间:2,8(大于等于2，小于等于8)
结果1,5（1是符合区间最左边的下标，5是符合区间最右边的下标）
要求时间复杂度要小于O(N)(不可以是O(N))

解法1:二分查找

private int[] array = {1,2,2,3,4,6};

private int min = -20;

private int max = 8;

public void binarySearch() {
    int left = binarySearch(array,min,true);
    int right = binarySearch(array,max,false);
    System.out.println(left +" "+ right);
}

private int binarySearch(int[] array,int target,boolean first){
    int length = array.length;
    if (length == 0) {
        return -1;
    }
    if (array[0] > max) {
        return -1;
    }
    if (array[length-1] < min) {
        return -1;
    }
    int left = 0,right = length -1;
    while (left <= right) {
        int mid = left + (right -left)/2;
        if (array[mid] > target) {
            right = mid - 1;
        }else if (array[mid] < target) {
            left = mid + 1;
        }else {
            if (first) {
                if (mid == 0 || array[mid - 1] != target) {
                    return mid;
                } else {
                    right = mid - 1;
                }
            } else {
                if (mid == 0 || array[mid + 1] != target) {
                    return mid;
                } else {
                    left = mid + 1;
                }
            }
        }
    }
    if (first) {
        return left;
    } else {
        return right;
    }
}

github源码