leetcode(105)从前序遍历和中序遍历构建二叉树

从前序遍历和中序遍历构建二叉树

class Solution {
    public HashMap<Integer,Integer> map = new HashMap<>();
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        int len = preorder.length;
        for(int i=0;i<len;++i){
            map.put(inorder[i],i);
        }
        TreeNode root = buildTree2(preorder, inorder, 0, len-1, 0);
        return root;
    }
    public TreeNode buildTree2(int[] preorder, int[] inorder, int start, int end, int first) {
        if(start>end) {
            return null;
        }  
        TreeNode root = new TreeNode(preorder[first]);
        int middle = map.get(preorder[first]);
        root.left = buildTree2(preorder,inorder,start,middle-1,first+1);
        root.right = buildTree2(preorder,inorder,middle+1,end,first+(middle-start)+1);
        return root;
    }
}