二分查找

查找一个数

int binarySearch(int[] nums, int target) {
 int left = 0; 
 int right = nums.length - 1; // 注意
 while(left <= right) {
 int mid = left + (right - left) / 2;
 if(nums[mid] == target)
 return mid; 
 else if (nums[mid] < target)
 left = mid + 1; // 注意
 else if (nums[mid] > target)
 right = mid - 1; // 注意
 }
 return -1;
}

查找左侧边界

int left_bound(int[] nums, int target) {
 if (nums.length == 0) return -1;
 int left = 0;
 int right = nums.length; // 注意
 
 while (left < right) { // 注意
 int mid = left + (right - left) / 2;
 if (nums[mid] == target) {
 right = mid;
 } else if (nums[mid] < target) {
 left = mid + 1;
 } else if (nums[mid] > target) {
 right = mid; // 注意
 }
 }
 return left;
}

查找右侧边界

int right_bound(int[] nums, int target) {
 if (nums.length == 0) return -1;
 int left = 0, right = nums.length;
 
 while (left < right) {
 int mid = left + (right - left) / 2;
 if (nums[mid] == target) {
 left = mid + 1; // 注意
 } else if (nums[mid] < target) {
 left = mid + 1;
 } else if (nums[mid] > target) {
 right = mid;
 }
 }
 return left - 1; // 注意
}