我正在尝试第一次出现数字5
。在这种情况下,答案应为2,但我在这里得到3。
public static void main(String[] args) {
int A[] = {1, 3, 5, 5, 5, 17, 20};
int index = BinarySearch(A, 5);
System.out.println(index);
}
public static int BinarySearch(int[] A, int x) {
int low = 0;
int high = A.length - 1;
while (low <= high) {
//(low + high) / 2
int mid = low + (high-low)/2; // more optimal -> low + (high-low)/2 (avoid integer overflow)
if (x == A[mid]) {
return mid;
} else if (x < A[mid]) {
high = mid - 1;
} else {
low = mid + 1;
}
}
return -1;
}
答案 0 :(得分:1)
当您找到要查找的值时,立即返回mid
索引,而不检查是否有较小的索引具有相同的值。
你必须继续搜索:
public static int BinarySearch(int[] A, int x) {
int low = 0;
int high = A.length - 1;
int mid = -1;
while (low <= high) {
mid = low + (high-low)/2;
if (x <= A[mid]) { // this ensures you keep searching for the first index having
// the number you are looking for
//
// changing x <= A[mid] to x < A[mid] will give you the
// last index having the number you are looking for instead
high = mid - 1;
} else {
low = mid + 1;
}
}
if (mid >= 0 && x == A[mid]) {
return mid;
}
return -1;
}