leetcode34--在排序数组中查找元素的第一个和最后一个位置

原题

给定一个按照升序排列的整数数组 nums,和一个目标值 target。找出给定目标值在数组中的开始位置和结束位置。

你的算法时间复杂度必须是 O(log n) 级别。

如果数组中不存在目标值,返回 [-1, -1]

示例1:

输入: nums = [5,7,7,8,8,10], target = 8
输出: [3,4]

示例2:

输入: nums = [5,7,7,8,8,10], target = 6
输出: [-1,-1]

解法

思想

二分查找,找到一个目标值时向两边继续查找最初和最末出现的地方。

代码

class Solution {
    public int[] searchRange(int[] nums, int target) {
        int[] pos = new int[2];
        int start = 0;
        int end = nums.length - 1;
        if(end == -1) return new int[]{-1,-1};
        int mid = start + (end-start)/2;
        while(nums[mid]!=target &&start<=end){
            mid = start + (end-start)/2;
            if(nums[mid]<target) start = mid + 1;
            else if(nums[mid] > target) end = mid -1;
        }
        if(nums[mid]!=target) return new int[]{-1,-1};
        int mid_cache = mid;
        int end_cache = end;
        end = mid;
        while(start<end){
            mid = start + (end-start)/2;
            if(nums[mid] == target) end = mid;
            else if(nums[mid] < target) start = mid+1;
        }
        pos[0] = start;
        start = mid_cache;
        end = end_cache;
        while(start<end){
            mid = start + (end-start+1)/2;
            if(nums[mid] == target) start = mid;
            else if(nums[mid] > target) end = mid-1;
        }
        pos[1] = end;
        return pos;       
    }
}

原创文章,作者:彭晨涛,如若转载,请注明出处:https://www.codetool.top/article/leetcode34-%e5%9c%a8%e6%8e%92%e5%ba%8f%e6%95%b0%e7%bb%84%e4%b8%ad%e6%9f%a5%e6%89%be%e5%85%83%e7%b4%a0%e7%9a%84%e7%ac%ac%e4%b8%80%e4%b8%aa%e5%92%8c%e6%9c%80%e5%90%8e%e4%b8%80%e4%b8%aa%e4%bd%8d/

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