We know,

Formal charge $=$ valence electrons $-$ non-bonding electrons $-\frac{\mathrm{bonding} \mathrm{electrons}}{2}$

Given structure is  

${\left[\begin{array}{l}\underset{..}{\stackrel{..}{\mathrm{O}}}=\underset{}{N}=\underset{..}{\stackrel{..}{O}}\end{array}\right]}^{+}$

Formal charge on the left most oxygen atom $=6-4-\frac{4}{2}=0$

Formal charge on the nitrogen atom in the middle $=5-0-\frac{8}{2}=+1$

Formal charge on the right most oxygen atom $=6-4-\frac{4}{2}=0$