二分查找排序数组中指定数所在区间

最新推荐文章于 2023-01-24 15:56:55 发布

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#二分查找

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本文介绍了解决LeetCode中寻找目标元素起始和结束位置问题的两种方法：使用二分查找法直接搜索和利用自定义的下界查找函数。通过具体的Java代码实现展示了如何高效地解决这一经典算法问题。

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import java.util.Arrays;

public class Solution {
    //解法1
    public int[] searchRange(int[] nums, int target) {
        if (nums == null || nums.length == 0)
            return new int[]{-1, -1};
        int i = 0;
        int j = nums.length - 1;
        int start = -1, end = -1;
        int mid;
        while (i < j) {
            mid = (i + j) / 2;
            if (target <= nums[mid]) {
                j = mid;
            } else {
                i = mid + 1;
            }
        }
        if (nums[i] == target) {
            start = i;
        } else {
            return new int[]{-1, -1};
        }

        j = nums.length - 1;
        while (i < j) {
            mid = (i + j + 1) / 2;
            if (target >= nums[mid]) {
                i = mid;
            } else {
                j = mid - 1;
            }
        }
        if (nums[i] == target) {
            end = i;
        }

        return new int[]{start, end};
    }

    //解法2
    public int[] searchRange2(int[] nums, int target) {
        if (nums == null || nums.length == 0)
            return new int[]{-1, -1};
        int start = lowerBound(nums, target);
        if (start >= 0 && start < nums.length && nums[start] == target)
            return new int[]{start, lowerBound(nums, target + 1) - 1};
        return new int[]{-1, -1};
    }

    public int lowerBound(int[] nums, int target) {
        int i = 0;
        int j = nums.length;
        int mid;
        while (i < j) {
            mid = (i + j) / 2;
            if (target <= nums[mid]) {
                j = mid;
            } else {
                i = mid + 1;
            }
        }
        return i;
    }

    public static void main(String[] args) {
        int[] nums = {5, 7, 7, 8, 8, 10};
        int target = 8;
        System.out.println(Arrays.toString(new Solution().searchRange(nums, target)));
    }

}