1.基本的二分查找
public static int binarySearch(int[] nums, int target) {
int left = 0;
int right = nums.length - 1;
if (target < nums[left] || target > nums[right]) {
return -1;
}
while (left <= right) {
int mid = left + (right - left) / 2;
if (nums[mid] < target) {
left = mid + 1;
} else if (nums[mid] > target) {
right = mid - 1;
} else if (nums[mid] == target) {
return mid;
}
// 注意"else if (nums[mid] == target)"該條件可不寫
// 直接寫else也是對的
}
return -1;
}
2.尋找左側邊界的二分查找
public static int leftBound(int[] nums, int target) {
int left = 0;
int right = nums.length - 1;
if (target < nums[left] || target > nums[right]) {
return -1;
}
while (left <= right) {
int mid = left + (right - left) / 2;
if (nums[mid] < target) {
left = mid + 1;
} else if (nums[mid] > target) {
right = mid - 1;
} else if (nums[mid] == target) {
// 不能返回,鎖定左邊界,繼續縮小右邊界
right = mid - 1;
}
// 上面的"right = mid - 1;"代碼體相同,
// 判斷條件可以合併爲else if (nums[mid] >= target)
}
return nums[left] == target ? left : -1;
}
3.尋找右側邊界的二分查找
public static int rightBound(int[] nums, int target) {
int left = 0;
int right = nums.length - 1;
if (target < nums[left] || target > nums[right]) {
return -1;
}
while (left <= right) {
int mid = left + (right - left) / 2;
if (nums[mid] < target) {
left = mid + 1;
} else if (nums[mid] > target) {
right = mid - 1;
} else if (nums[mid] == target) {
// 不能返回,鎖定右邊界,繼續縮小左邊界
left = mid + 1;
}
// 上面的"left = mid + 1;"代碼體相同,
// 判斷條件可以合併爲else if (nums[mid] <= target)
}
return nums[right] == target ? right : -1;
}
4.說明
這是一個Java實現的二分查找法,
包括其基本的二分查找法及其兩個變種,
找到最左邊第1個等於target的二分法,
找到最右邊最後一個等於target的二分法。
分析上面的代碼可以發現
其區別在於nums[mid] == target時的處理,
基本二分法找到即會立刻返回;
查找左邊界時,需要不斷收縮右邊界,直到找到最左邊;
查找右邊界時,需要不斷收縮左邊界,直到找到最右邊。
5.測試代碼
package edu.yuwen.algorithms.binarysearch;
public class BinarySearch {
public static void main(String[] args) {
int nums[] = { 1, 2, 3, 3, 3, 4, 5, 6 };
int mid = binarySearch(nums, 3);
System.out.println("1.binary search=" + mid);
mid = leftBound(nums, 3);
System.out.println("2.left bound=" + mid);
mid = rightBound(nums, 3);
System.out.println("3.right bound=" + mid);
}
}
輸出結果如下:
1.binary search=3
2.left bound=2
3.right bound=4