Java实现自定义线段树(附注解)

川长思鸟来 2021-12-13 20:11 98阅读 0赞

## **导读:** ##

本文分为三部分:  
1.文章介绍  
2.实现代码及介绍  
3.测试代码及测试结果

**一.文章介绍**  
本文主要是使用了Java语言实现一个将数组构造为一个可以更新及查询的线段树结构, 线段树主要是用于解决线段和区间问题, 是一种高级的数据结构.

**二.实现代码**

自定义生成方式接口:

//  使线段树可以自定义生成方式
    public interface Merge<E> {
        E merge(E a, E b);
    }

线段树类代码:

public class SegmentTree<E> {
    
        private E[] tree;
        private E[] data;
        private Merge<E> merge;
    
        public SegmentTree(E[]arr, Merge<E> merge){
            this.merge = merge;
            data = (E[])new Object[arr.length];
            for (int i = 0; i < arr.length; i++) {
                data[i] = arr[i];
            }
            tree = (E[])new Object[4 * arr.length];
            buildSegmentTree(0, 0, data.length-1);
        }
    	
    	// 构造线段树结构
        private void buildSegmentTree(int treeIndex, int l, int r) {
    
            if (l == r){
                tree[treeIndex] = data[l];
                return;
            }
    
            int leftChildIndex = leftChild(treeIndex);
            int rightChildIndex = rightChild(treeIndex);
    
            int mid = l + (r - l) / 2;
            buildSegmentTree(leftChildIndex,l,mid);
            buildSegmentTree(rightChildIndex,mid + 1,r);
    
            tree[treeIndex] = merge.merge(tree[leftChildIndex],tree[rightChildIndex]);
    
        }
    
        public int getSize(){
            return data.length;
        }
    
        public E get(int index){
            if (index < 0 && index > data.length-1)
                throw new IllegalArgumentException("Index is Illegal");
            return data[index];
        }
    
        // 返回完全二叉树的数组表示中, 一个索引所表示的元素的左孩子节点的索引
        public int leftChild(int index){
            return 2*index + 1;
        }
    
        // 返回完全二叉树的数组表示中, 一个索引所表示的元素的右孩子节点的索引
        public int rightChild(int index){
            return 2*index + 2;
        }
    
        // 在线段树中搜索区间[queryL...queryR]的值
        public E query(int queryL, int queryR){
            if (queryL < 0 || queryL >= data.length ||
                    queryR < 0 || queryR >= data.length || queryL > queryR)
                throw new IllegalArgumentException("Error");
            return query(0, 0, data.length-1, queryL ,queryR);
        }
    
        // 在以treeIndex为根的线段树中[l...r]的范围里, 搜索区间[queryL...queryR]的值
        private E query(int treeIndex, int l, int r, int queryL, int queryR) {
    
            if (queryL == l && queryR == r)
                return tree[treeIndex];
    
            int mid = l + (r - l) / 2;
    
            int leftChildIndex = leftChild(treeIndex);
            int rightChildIndex = rightChild(treeIndex);
    
            if (queryL >= mid + 1)
                return query(rightChildIndex,mid + 1, r, queryL
                , queryR);
            else if (queryR <= mid)
                return query(leftChildIndex,l, mid, queryL
                        , queryR);
    
            E leftRes = query(leftChildIndex, l, mid, queryL, mid);
            E rightRes = query(rightChildIndex, mid + 1, r, mid + 1, queryR);
            return merge.merge(leftRes, rightRes);
    
        }
    
        // 将index位置的值, 更新为e
        public void set(int index, E e){
    
            if (index < 0 || index > data.length)
                throw new IllegalArgumentException("Index is illegal");
    
            data[index] = e;
            set(0,0,data.length-1,index,e);
        }
    
        private void set(int treeIndex, int l, int r, int index, E e) {
            if (l == r){
                tree[treeIndex] = e;
                return;
            }
    
            int mid = l + (r - l) / 2;
            int leftChildIndex = leftChild(treeIndex);
            int rightChildIndex = rightChild(treeIndex);
    
            if (index >= mid + 1)
                set(rightChildIndex, mid + 1, r, index, e);
            else
                set(leftChildIndex, l,mid,index,e);
    
            tree[treeIndex] = merge.merge(tree[leftChildIndex], tree[rightChildIndex]);
    
        }
    
    
        @Override
        public String toString() {
            StringBuilder res = new StringBuilder();
            res.append('[');
            for (int i = 0; i < tree.length; i++) {
                if (tree[i] != null)
                    res.append(tree[i]);
                else
                    res.append("null");
                if (i != tree.length -1)
                    res.append(',');
                else
                    res.append(']');
            }
            return res.toString();
        }
    }

**三.测试代码**

public static void main(String[] args) {
            Integer[] nums = {-2, 0, 3, -5, 2, -1};
            SegmentTree<Integer> segmentTree = new SegmentTree<>(nums, (a, b) -> a + b);
    
    
            System.out.println(segmentTree.query(2, 5));
            segmentTree.set(2, 10);
            System.out.println(segmentTree.query(2, 5));
            System.out.println(segmentTree.toString());
        }

测试结果:  
![在这里插入图片描述][20190707110902867.png]

[20190707110902867.png]: /images/20211213/5fee6cf5bcc649c5891a78fd574c69e7.png