线段树

public class SegmentTree<E> {
	
	private E[] tree;
	private E[] data;
	private Merger<E> 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[4 * arr.length];
		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) {
		
		if(l == r) {
			tree[treeIndex] = data[l];
			return ;
		}
		
		int leftTreeIndex = leftChild(treeIndex);
		int rightTreeIndex = rightChild(treeIndex);
		
		int mid = l + (r - l) / 2;
		buildSegmentTree(leftTreeIndex, l, mid);
		buildSegmentTree(rightTreeIndex, mid + 1, r);
		
		tree[treeIndex] = merger.merge(tree[leftTreeIndex], tree[rightTreeIndex]);
	}
	
	public int getSize() {
		return data.length;
	}
	
	public 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;
	}
	
	// 返回区间[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("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(l == queryL && r == queryR) {
			return tree[treeIndex];
		}
		
		int mid = l + (r - l) / 2;
		// treeIndex的结点分为[l..mid]和[mid + 1..r]两部分
		
		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);
		}
		
		E leftResult = query(leftTreeIndex, l, mid, queryL, mid);
		E rightResult = query(rightTreeIndex, mid + 1, r, mid + 1, queryR);
		
		return merger.merge(leftResult, rightResult);
	}
	
	@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(", ");
			}
		}
		res.append(‘]‘);
		return res.toString();
	}
}

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

public class Main {

	public static void main(String[] args) {

		Integer[] nums = {-2, 0, 3, -5, 2, -1};
//		SegmentTree<Integer> segTree = new SegmentTree<>(nums, new Merger<Integer>() {
//			public Integer merge(Integer a, Integer b) {
//				return a + b;
//			}
//		});
		SegmentTree<Integer> segTree = new SegmentTree<>(nums, (a, b)->a + b);
//		System.out.println(segTree);
		System.out.println(segTree.query(0, 2));
	}

}