Re: Previous large element
So for each element in an array you want to find the closest bigger element to the left?
Originally Posted by abhi1988srivastava
I think this can be accomplished in O(N) with the use of a stack.
You scan the array in reverse (from right to left) and consider each array element in turn. If an element is bigger than the top-of-stack it's a hit. You've found the first bigger element to the left of the top-of-stack element. The stack is popped. If the element is bigger than the new top-of-stack too it's another hit so the stack is popped again, etcetera. Then the element is pushed and the next array element in turn is considered.
Each of the N array elements is pushed and popped once at the most. Since push/pop are O(1) operations the total complexity will be O(N).
Key to this low complexity is that a stack can be used rather than a tree (which would've resulted in O(N * logN)). And this is possible because the structure of the algorithm keeps the stack in sorted order automagically, avoiding explicit ordering.