Segmemt Tree Build II

35% Accepted

The structure of Segment Tree is a binary tree
which each node has two attributes start and end denote an segment / interval.

start and end are both integers, they should be assigned in following rules:

The root's start and end is given by build method.
The left child of node A has start=A.left, end=(A.left + A.right) / 2.
The right child of node A has start=(A.left + A.right) / 2 + 1, end=A.right.
if start equals to end, there will be no children for this node.
Implement a build method with a given array, so that we can create a corresponding segment tree with every node value represent the corresponding interval max value in the array, return the root of this segment tree.

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Example
Given [3,2,1,4]. The segment tree will be:

                 [0,  3] (max = 4)
                  /            \
        [0,  1] (max = 3)     [2, 3]  (max = 4)
        /        \               /             \
[0, 0](max = 3)  [1, 1](max = 2)[2, 2](max = 1) [3, 3] (max = 4)
Clarification
Segment Tree (a.k.a Interval Tree) is an advanced data structure which can support queries like:

which of these intervals contain a given point
which of these points are in a given interval
See wiki: Segment Tree Interval Tree

Tags Expand

Segment Tree

思路

• Segment Tree Build稍微变形

/**
 * Definition of SegmentTreeNode:
 * public class SegmentTreeNode {
 *     public int start, end, max;
 *     public SegmentTreeNode left, right;
 *     public SegmentTreeNode(int start, int end, int max) {
 *         this.start = start;
 *         this.end = end;
 *         this.max = max
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {

    public SegmentTreeNode build(int[] A) {
        int size = A.length - 1;
        return build(0, size, A);
    }

    public SegmentTreeNode build(int start, int end, int[] A) {
        // write your code here
        if (start > end) {
            return null;
        }
        int max = findmax(A, start, end);
        if (start == end) {
            return new SegmentTreeNode(start, end, max);
        }
        SegmentTreeNode root = new SegmentTreeNode(start, end, max);
        root.left = build(start, (start + end) /2, A);
        root.right = build((start + end) / 2 + 1, end, A);

        return root;
    }

    public int findmax(int[] A, int start, int end) {
        int max = Integer.MIN_VALUE;
        for (int i = start; i <= end; i++) {
            if (A[i] > max) {
                max = A[i];
            }
        }
        return max;
    }
}