We can postpone updating children we don ' t ▁ ▁ need ▁ their ▁ new ▁ values ▁ now . ▁ ▁ Since ▁ we ▁ are ▁ not ▁ yet ▁ updating ▁ children ▁ of ▁ ' node ',  we need to set lazy flags for the children
lazy [ node << 1 ] = ! lazy [ node << 1 ] ; lazy [ 1 + ( node << 1 ) ] = ! lazy [ 1 + ( node << 1 ) ] ; } }