105. Construct Binary Tree from Preorder and Inorder Travers

 1 class Solution {
 2     public TreeNode buildTree(int[] preorder,int[] inorder) {
 3         return helper(0,inorder.length - 1,preorder,inorder);
 4         
 5     }
 6     
 7     public TreeNode helper(int preStart,int inStart,int inEnd,int[] preorder,int[] inorder) {
 8         if(preStart > preorder.length - 1 || inStart > inEnd) {
 9             return null;
10         }
11         TreeNode root = new TreeNode(preorder[preStart]);
12         int rootIndex = 0;
13         for(int i = inStart; i <= inEnd; i++) {  // i 不是 =0 是等于 = inStart
14             if(root.val == inorder[i]) {
15                 rootIndex = i;
16             }
17         }
18         root.left = helper(preStart + 1,inStart,rootIndex - 1,inorder);
19         root.right = helper(preStart + rootIndex - inStart + 1,rootIndex + 1,inEnd,inorder);
20         return root;
21     }
22 }