Trapping Rain Water | Java implementation of the approach ; Function to return the maximum water that can be stored ; Let the first element be stored as previous , we shall loop from index 1 ; To store previous wall 's index ; To store the water until a larger wall is found , if there are no larger walls then delete temp value from water ; If the current wall is taller than the previous wall then make current wall as the previous wall and its index as previous wall 's index  for the subsequent loops ; Because larger or same height wall is found ; Since current wall is shorter than the previous , we subtract previous wall ' s ▁ height ▁ from ▁ the ▁ current ▁ wall ' s height and add it to the water ; Store the same value in temp as well If we dont find any larger wall then we will subtract temp from water ; If the last wall was larger than or equal to the previous wall then prev_index would be equal to size of the array ( last element ) If we didn 't find a wall greater than or equal  to the previous wall from the left then  prev_index must be less than the index  of the last element ; Temp would ' ve ▁ stored ▁ the ▁ water ▁ collected ▁ ▁ from ▁ previous ▁ largest ▁ wall ▁ till ▁ the ▁ end ▁ ▁ of ▁ array ▁ if ▁ no ▁ larger ▁ wall ▁ was ▁ found ▁ then ▁ ▁ it ▁ has ▁ excess ▁ water ▁ and ▁ remove ▁ that ▁ ▁ from ▁ ' water ' var ; We start from the end of the array , so previous should be assigned to the last element ; Loop from the end of array up to the ' previous ▁ index ' which would contain the " largest ▁ wall ▁ from ▁ the ▁ left " ; Right end wall will be definitely smaller than the ' previous ▁ index ' wall ; Return the maximum water ; Driver code
class GFG { public static int maxWater ( int arr [ ] , int n ) { int size = n - 1 ; int prev = arr [ 0 ] ; int prev_index = 0 ; int water = 0 ; int temp = 0 ; for ( int i = 1 ; i <= size ; i ++ ) { if ( arr [ i ] >= prev ) { prev = arr [ i ] ; prev_index = i ; temp = 0 ; } else { water += prev - arr [ i ] ; temp += prev - arr [ i ] ; } } if ( prev_index < size ) { water -= temp ; prev = arr [ size ] ; for ( int i = size ; i >= prev_index ; i -- ) { if ( arr [ i ] >= prev ) { prev = arr [ i ] ; } else { water += prev - arr [ i ] ; } } } return water ; } public static void main ( String [ ] args ) { int arr [ ] = { 0 , 1 , 0 , 2 , 1 , 0 , 1 , 3 , 2 , 1 , 2 , 1 } ; int n = arr . length ; System . out . print ( maxWater ( arr , n ) ) ; } }