text: If the input index is in range of this node , then update the value of the node and its children
code: st [ si ] = st [ si ] + diff ; if ( se != ss ) { int mid = getMid ( ss , se ) ; updateValueUtil ( st , ss , mid , i , diff , 2 * si + 1 ) ; updateValueUtil ( st , mid + 1 , se , i , diff , 2 * si + 2 ) ; } }