leetcode---Contains Duplicate III

Given an array of integers, find out whether there are two distinct indices i and j in the array such that
the difference between nums[i] and nums[j] is at most t and
the difference between i and j is at most k.

class Solution {
public:
bool containsNearbyAlmostDuplicate(vector<int>& nums, int k, int t)
{
int n = nums.size();
map<int, bool> m;
int first = 0;
for(int i=0; i<n; i++)
{
if(i - first > k && m[nums[first]] == true)
{
m.erase(nums[first]);
++first;
}
auto it = m.lower_bound(nums[i]-t);
if(it != m.end() && abs(nums[i] - it->first) <= t)
return true;
m[nums[i]] = true;
}
return false;
}
};