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

这个是用递归的方式写的，非递归也可以写。

注意判断的时候可能出现数组越界的问题，所以要判断mid - 1 < 0和mid + 1 >= nums.size()，即数组的头和数组的尾

如果不判断[1],1这种就会报错

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        vector<int> result;
        int first = find_first(nums,target,0,nums.size() - 1);
        int last = find_last(nums,target,0,nums.size() - 1); 
        if(first != -1 && last != -1){
            result.push_back(first);
            result.push_back(last);
        }
        else{
            result.push_back(-1);
            result.push_back(-1);
        }
        return result;
    }
    int find_first(vector<int>& nums,int target,int start,int end){
        if(start > end)
            return -1;
        int mid = (start + end)/2;
        if(nums[mid] < target)
            return find_first(nums,target,mid+1,end);
        else if(nums[mid] > target)
            return find_first(nums,target,start,mid-1);
        else{
            if(mid - 1 < 0 || nums[mid - 1] != target)
                return mid;
            else
                return find_first(nums,target,start,mid-1);
        }
    }
    int find_last(vector<int>& nums,int target,int start,int end){
        if(start > end)
            return -1;
        int mid = (start + end)/2;
        if(nums[mid] < target)
            return find_last(nums,target,mid+1,end);
        else if(nums[mid] > target)
            return find_last(nums,target,start,mid-1);
        else{
            if(mid + 1 >= nums.size() || nums[mid + 1] != target)
                return mid;
            else
                return find_last(nums,target,mid+1,end);
        }
    } 
};