From e15492b0b69ca06d8d90057093237de8767bf784 Mon Sep 17 00:00:00 2001
From: dbcai <dbcai@qq.com>
Date: Tue, 26 Apr 2022 22:08:25 +0800
Subject: [PATCH] =?UTF-8?q?=E6=B7=BB=E5=8A=A0=20=E5=89=8D=E5=BA=8F.ACM?=
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 =?UTF-8?q?=E5=8F=89=E6=A0=91.md=20Java=E7=89=88=E6=9C=AC?=
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---
 .../前序/ACM模式如何构建二叉树.md  | 53 +++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/problems/前序/ACM模式如何构建二叉树.md b/problems/前序/ACM模式如何构建二叉树.md
index 28c4b6f7..07674b31 100644
--- a/problems/前序/ACM模式如何构建二叉树.md
+++ b/problems/前序/ACM模式如何构建二叉树.md
@@ -217,6 +217,59 @@ int main() {
 ## Java 
 
 ```Java
+public class Solution {
+    // 节点类
+    static class TreeNode {
+        // 节点值
+        int val;
+
+        // 左节点
+        TreeNode left;
+
+        // 右节点
+        TreeNode right;
+
+        // 节点的构造函数(默认左右节点都为null)
+        public TreeNode(int x) {
+            this.val = x;
+            this.left = null;
+            this.right = null;
+        }
+    }
+    
+    /**
+     * 根据数组构建二叉树
+     * @param arr 树的数组表示
+     * @return 构建成功后树的根节点
+     */
+    public TreeNode constructBinaryTree(final int[] arr) {
+        // 构建和原数组相同的树节点列表
+        List<TreeNode> treeNodeList = arr.length > 0 ? new ArrayList<>(arr.length) : null;
+        TreeNode root = null;
+        // 把输入数值数组，先转化为二叉树节点列表
+        for (int i = 0; i < arr.length; i++) {
+            TreeNode node = null;
+            if (arr[i] != -1) { // 用 -1 表示null
+                node = new TreeNode(arr[i]);
+            }
+            treeNodeList.add(node);
+            if (i == 0) {
+                root = node;
+            }
+        }
+        // 遍历一遍，根据规则左右孩子赋值就可以了
+        // 注意这里 结束规则是 i * 2 + 2 < arr.length，避免空指针
+        for (int i = 0; i * 2 + 2 < arr.length; i++) {
+            TreeNode node = treeNodeList.get(i);
+            if (node != null) {
+                // 线性存储转连式存储关键逻辑
+                node.left = treeNodeList.get(2 * i + 1);
+                node.right = treeNodeList.get(2 * i + 2);
+            }
+        }
+        return root;
+    }
+}
 ```