geometric-applications-of-bst.md

Geometric Applications of BST

1-D Range Search

This is the extension for symbol table.

• Insert key-value pair.
• Search for key k.
• Delete key k.
• Range search: Find all keys between k1 & k2.
• Range count: Number of keys between k1 & k2.

Implementation

• Un Ordered List: Fast Insertion and slow search
• Ordered List: Slow insert, binary search for k1 and k2 to do range search.

BST Implementation

Example1:

Java code to find the number of keys between low and high

// Running time is proportional to log(n)
public int size(key low, key high) {
    if(contains(high)){
        return rank(high) - rank(low) + 1;
    } else {
        return rank(high) - rank(low);
    }
}

Example 2:

Line Segment Intersection

Given N horizontal and vertical line segments, find all intersections.

Sweep-line algorithm

Sweep vertical line between left to right and record your observations

Algorithm:

• x-coordinates define events.
• h-segment (left endpoint): insert y-coordinate into BST.
• h-segment (left endpoint): insert y-coordinate into BST.
• v-segment: Range search for interval of y-end points.

Explanation:

• Put x-coordinate in PQ (or Sort ) . -------------> N * log(N)
• Insert y-coordinate into the BST . -------------> N * log(N)
• Remove y-coordinate from the BST -------------> N * log(N)
• Range Searches in BST -------------> N * log(N) + R

1D Interval Search Trees

Data structure to hold the set of overlapping intervals.

Create BST, where each node stores an interval (lo, hi).

• Use left endpoint as BST key.
• Store max endpoint in subtree rooted at node.

Algorithm

• If interval in node intersects query interval, return it.
• Else if left subtree is null, go right.
• Else if max endpoint in left subtree is less than lo, go right.
• Else go left.

Java Implementation

..............

 TreeNode x = root;
 
 while(x!=null){
     if(x.interval.insersects(lo,hi)){
         return x.interval;
     }else if(x.left == null) {
         x = x.right;
     }else if(x.left.max < lo){
         x = x.right;
     }else {
         x = x.left;
     }
 }

return null;

..............

Interval Search Tree Analysis

Implementation: Use a red-black BST ( easy to maintain auxiliary information using log N extra work per op ) to guarantee performance.

Orthogonal rectangle intersection

Goal: Find all intersections among a set of N orthogonal rectangles.

Here we use the same principle sweep line algorithm. Sweep one vertical line from left to right. when you encounter left end point insert y interval. and repeat the interval search until you get the right end point.

Algorithm

• x-coordinates of left and right endpoints define events.
• Maintain set of rectangles that intersect the sweep line in an interval search tree (using y-intervals of rectangle).
• Left endpoint: Interval search for y-interval of rectangle; insert y-interval.
• Right endpoint: Remove y-interval.

Analysis

• Put x-coordinate in PQ (or Sort ) . -------------> N * log(N)
• Insert y-coordinate into the BST . -------------> N * log(N)
• Reomve y-coordinate from the BST -------------> N * log(N)
• Interval Search for y Coordinates -------------> N * log(N) + R * log(N)

Summary