LeetCode307中等问题，自写线段树结构解法

class NumArray {
    public interface Merger<E> {
        E merge(E a, E b);
    }
    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) {
            // TODO 自动生成的方法存根
            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];
        }
        
        // 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
        private 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
        private int rightChild(int index) {
            return 2 * index + 2;
        }
        
        // 返回区间[queryL, queryR]的值
        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treeID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;
            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);
        }
        
        // 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);
        }
        
        // 在以treeIndex为根的线段树中更新index的值为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 leftTreeIndex = leftChild(treeIndex);
            int rightTreeIndex = rightChild(treeIndex);
            if(index >= mid + 1)
                set(rightTreeIndex, mid + 1, r, index, e);
            else // index <= mid
                set(leftTreeIndex, l, mid, index, e);
            
            tree[treeIndex] = merger.merge(tree[leftTreeIndex], tree[rightTreeIndex]);
        }
        
        @Override
        public String toString() {
            StringBuilder res = new StringBuilder();
            res.append('[');
            for (int i = 0; i < data.length; i ++) {
                if(tree[i] != null)
                    res.append(tree[i]);
                
                else
                    res.append("null");
                
                if(i != tree.length);
            }
            return null;
        }
    }
    private SegmentTree<Integer> segmentTree;
	public NumArray(int[] nums) {
		if(nums.length > 0) {
			Integer[] data = new Integer[nums.length];
			for(int i = 0; i < nums.length ; i ++)
				data[i] = nums[i];
			segmentTree = new SegmentTree<>(data, (a, b) -> a + b);
		}
    }
	
	public void update(int index, int val) {
		if(segmentTree == null)
			throw new IllegalArgumentException("Segment Tree is null");
		segmentTree.set(index, val);
	}
    
    public int sumRange(int i, int j) {
    	
    	if(segmentTree == null)
    		throw new IllegalArgumentException("Segment Tree is null");
    	
    	return segmentTree.query(i, j);
    }
}