線段樹

數組轉成線段樹，遞歸實現

線段樹這種數據結構可讓統計更快，好比任何多數據求和操做能夠在LOG（N）的複雜度完成。

public class SegmentTree<E> {

    private E[] data;
    private E[] tree;
    private Marger<E> marger;
    public SegmentTree(E[] arr, Marger<E> marger) {
        data=(E[]) new Object[arr.length];
        this.marger = marger;
        for (int i = 0; i < arr.length; i++) {
            E e=arr[i];
            data[i]=e;
        }

        tree= (E[]) new Object[4*arr.length];
        buildSegmentTree(0,0,data.length-1);

    }

    //zai treeindex位置，建立表示區間【l,r】的線段樹
    private void buildSegmentTree(int treeIndex, int l, int r) {
        if (l==r){
            tree[treeIndex]=data[l];
            return;
        }

        int leftindex=leftChild(treeIndex);
        int rightindex=rightChild(treeIndex);
        int mid=l+(r-l)/2;
        buildSegmentTree(leftindex,l,mid);
        buildSegmentTree(rightindex,mid+1,r);
        tree[treeIndex]= marger.marger(tree[leftindex],tree[rightindex]);


    }

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

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

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

    public String toString(){
            StringBuilder res = new StringBuilder();
        for (int i = 0; i < tree.length; i++) {
            if (tree[i]!=null)
                res.append(tree[i]);
            else res.append("null");
            res.append("+");
        }
            return res.toString();

        }

    public static void main(String[] args) {

        Integer[] nums={-2,0,3,-5,2,-1};
        SegmentTree<Integer> segmentTree=new SegmentTree<Integer>(nums, (a,b)->a+b);
        System.out.println(segmentTree.toString());
    }
    
    
}複製代碼