Update 二叉树相关.md

blogs/Algorithm/剑指 Offer/二叉树相关.md

@@ -33,3 +33,35 @@ class Solution {
 }
 ```
 
+```java
+class Solution {
+
+    public TreeNode buildTree(int[] preorder, int[] inorder) {
+        if (preorder == null || preorder.length == 0) {
+            return null;
+        }
+        TreeNode root = new TreeNode(preorder[0]);
+        int length = preorder.length;
+        Stack<TreeNode> stack = new Stack<>();
+        stack.push(root);
+        int inorderIndex = 0;
+        for (int i = 1; i < length; i++) {
+            int preorderVal = preorder[i];
+            TreeNode node = stack.peek();
+            if (node.val != inorder[inorderIndex]) {
+                node.left = new TreeNode(preorderVal);
+                stack.push(node.left);
+            } else {
+                while (!stack.isEmpty() && stack.peek().val == inorder[inorderIndex]) {
+                    node = stack.pop();
+                    inorderIndex++;
+                }
+                node.right = new TreeNode(preorderVal);
+                stack.push(node.right);
+            }
+        }
+        return root;
+    }
+}
+```
+