【LeetCode】34. Find First and Last Position of Element in Sorted Array . 查找有序数组中元素的第一个和最后一个位置

题目

34. Find First and Last Position of Element in Sorted Array

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思路

二分，注意结果是取l还是r，这样可以求出大于target的第一个和小于target的第一个

代码

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        vector<int> v;
        int n=nums.size();
        if(n==0 || target>nums[n-1] || target<nums[0]){
            v.push_back(-1);
            v.push_back(-1);
            return v;
        }
        
        int l=0,r=n-1,mid;
        while(l<=r){
            mid=(l+r)/2;
            if(nums[mid]>=target)r=mid-1;
            else l=mid+1;
        }
        if(nums[r+1]==target)v.push_back(r+1);
        else v.push_back(-1);
        l=0;r=n-1;
        while(l<=r){
            mid=(l+r)/2;
            if(nums[mid]<=target)l=mid+1;
            else r=mid-1;
        }
        if(l!=0&&nums[l-1]==target)v.push_back(l-1);
        else v.push_back(-1);
        return v;
    }
};