105. 从前序与中序遍历序列构造二叉树

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    Map<Integer,Integer> map;
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        map = new HashMap<>();
        for(int i = 0;i < inorder.length;i++){
            map.put(inorder[i],i);
        }
        //分治
        return help(preorder,0,preorder.length,inorder,0,inorder.length);
        
    }
    
    private TreeNode help(int[] pre,int preI,int preJ,
                            int[] in,int inI,int inJ){
        if(preI >= preJ) return null;
        //preI为root
        int num = pre[preI];
        int index = map.get(num);
        TreeNode root = new TreeNode(num);
        TreeNode left = help(pre,preI + 1,preI + 1 + index - inI ,in,inI,index);
        TreeNode right = help(pre,preI + 1 + index - inI, preJ ,in,index + 1,inJ);
        root.left = left;
        root.right = right;
        return root;
    }
}