线段树的初始化（递归）

java8中的BinaryOperator简化代码

src/main/java/com/ximo/datastructuresinaction/segmenttree/SegmentTree.java

@@ -0,0 +1,76 @@
+package com.ximo.datastructuresinaction.segmenttree;
+
+import java.util.Arrays;
+import java.util.function.BinaryOperator;
+
+import static java.util.stream.Collectors.joining;
+
+/**
+ * @author xikl
+ * @date 2019/1/7
+ */
+public class SegmentTree<E> {
+
+    /** 内部维护一个数组来存储数据 */
+    private E[] data;
+
+    /** 内部维护一个树 */
+    private E[] tree;
+
+    private BinaryOperator<E> merger;
+
+    @SuppressWarnings("unchecked")
+    public SegmentTree(E[] arr, BinaryOperator<E> merger) {
+        data = (E[]) Arrays.stream(arr).toArray();
+        tree = (E[]) new Object[4 * data.length];
+        buildSegmentTree(0, 0, data.length);
+        this.merger = merger;
+    }
+
+    /**
+     * 创建一个起始索引位置 在left 和 right 的一个 线段树 如 [0, data.length]
+     * 闭区间
+     *
+     * @param treeIndex 树的起始索引
+     * @param left 左边界
+     * @param right 右边界
+     */
+    private void buildSegmentTree(int treeIndex, int left, int right) {
+        if (left == right) {
+            tree[treeIndex] = data[left];
+            return;
+        }
+
+        int leftChildIndex = getLeftChildIndex(treeIndex);
+        int rightChildIndex = getRightChildIndex(treeIndex);
+        // 获得中间值 防止 内存溢出
+        int mid = left + (right - left) / 2;
+        buildSegmentTree(leftChildIndex, left, mid);
+        buildSegmentTree(rightChildIndex, mid + 1, right);
+        tree[treeIndex] = merger.apply(tree[leftChildIndex], tree[rightChildIndex]);
+    }
+
+    public E get(int index) {
+        if (index < 0 || index > data.length) {
+            throw new IndexOutOfBoundsException("index-out-of-bounds");
+        }
+        return data[index];
+    }
+
+    public int getSize() {
+        return data.length;
+    }
+
+    public int getLeftChildIndex(int index) {
+        return 2 * index + 1;
+    }
+
+    public int getRightChildIndex(int index) {
+        return 2 * index + 2;
+    }
+
+    @Override
+    public String toString() {
+        return Arrays.stream(tree).map(Object::toString).collect(joining("", "", ""));
+    }
+}