SegmentTree

Merger.java

public interface Merger<E> {
    E merge(E a, E b);
}

SegmentTree.java

import java.util.Arrays;

public class SegmentTree<E> {

    private E[] data;
    private E[] tree;
    private Merger merger;

    public SegmentTree(E[] arr, Merger<E> merger) {
        this.merger = merger;
        data = (E[]) new Object[arr.length];
        for (int i = 0; i < arr.length; i++) {
            data[i] = arr[i];
        }
        tree = (E[]) new Object[arr.length * 4];
        buildSegmentTree(0, 0, data.length - 1);
    }

    // 在 treeIndex 的位置创建表示区间[l..r]的线段树
    private void buildSegmentTree(int treeIndex, int l, int r) {
        // 先考虑递归到底
        if (l == r) {
            tree[treeIndex] = data[l];
            return;
        }

        int leftTreeIndex = leftChild(treeIndex);
        int rightTreeIndex = rightChild(treeIndex);
        int mid = l + (r - l) / 2;
        // 创建左右子树
        buildSegmentTree(leftTreeIndex, l, mid);
        buildSegmentTree(rightTreeIndex, mid + 1, r);

        tree[treeIndex] = (E) merger.merge(tree[leftTreeIndex], tree[rightTreeIndex]);
    }

    public int getSize() {
        return data.length;
    }

    private E get(int index) {
        if (index < 0 || index >= data.length) {
            throw new IllegalArgumentException("Index is illegal.");
        }
        return data[index];
    }

    private int leftChild(int index) {
        return 2 * index + 1;
    }

    private int rightChild(int index) {
        return 2 * index + 2;
    }

    public E query(int queryL, int queryR) {
        if (queryL < 0 || queryL >= data.length || queryR < 0 || queryR >= data.length) {
            throw new IllegalArgumentException("Index is illegal");
        }

        return query(0, 0, data.length + 1, queryL, queryR);
    }

    // 在以 treeIndex 为根的线段树中[l..r]的返回内，搜区间[queryL,queryR]的值
    private E query(int treeIndex, int l, int r, int queryL, int queryR) {
        // 先考虑查询到底的情况
        if (l == queryL && r == queryR) {
            System.out.println("Query " + treeIndex + " == " + tree[treeIndex]);
            return tree[treeIndex];
        }
        int mid = l + (r - l) / 2;
        int leftTreeIndex = leftChild(treeIndex);
        int rightTreeIndex = rightChild(treeIndex);
        if (queryL >= mid + 1) {
            return query(rightTreeIndex, mid + 1, r, queryL, queryR);
        } else if (queryR <= mid) {
            return query(leftTreeIndex, l, mid, queryL, queryR);
        } else {
            // 一部分落在左孩子，一部分落在右孩子
            E leftRect = query(leftTreeIndex, l, mid, queryL, mid);
            E rightRect = query(rightTreeIndex, mid + 1, r, mid + 1, queryR);
            return (E) merger.merge(leftRect, rightRect);
        }
    }
    
    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;
        }
        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] = (E) merger.merge(tree[leftChildIndex], tree[rightChildIndex]);
    }

    @Override
    public String toString() {
        return "SegmentTree{" +
                "data=" + Arrays.toString(data) +
                ", tree=" + Arrays.toString(tree) +
                ", merger=" + merger +
                '}';
    }
}