Looking for $2019$ among the rest.

The first row ends with 1.
The second row ends with 3.
The third row ends with 6.
The fourth row ends with 10.
In general, the $n$ -th row ends with $\Delta_n = \frac {n(n+1)}{2}$ .

$\frac {n(n+1)}{2}=2019 \Rightarrow n \approx 63$

$\Delta_{63} = \frac {63\cdot(63+1)}{2} = 2016$

This means that the 63rd row ends with 2016, so 2019 is the third number in row 64 .

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$\Delta_n$ is the $n$ -th triangular number .