构造二叉树

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        Map<Integer,Integer> map = new HashMap<>();
        for(int i = 0;i < preorder.length;i++) map.put(inorder[i],i);
        return help(preorder,0,preorder.length,inorder,0,preorder.length,map);
    }
    private TreeNode help(int[] preorder,int start,int end,
                          int[] inorder,int start2,int end2,Map<Integer,Integer> map){
        if(start >= end) return null;
        TreeNode root = new TreeNode(preorder[start]);
        int loc = map.get(preorder[start]);
        root.left = help(preorder,start+1,start + loc - start2 + 1,inorder,start2,loc,map);
        root.right = help(preorder,start + loc - start2 + 1,end,inorder,loc + 1,end2,map);

        return root;
    }
}