剑指offer 04：重构二叉树

题目描述

输入某二叉树的前序遍历和中序遍历的结果，请重建出该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。例如输入前序遍历序列{1,2,4,7,3,5,6,8}和中序遍历序列{4,7,2,1,5,3,8,6}，则重建二叉树并返回。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode reConstructBinaryTree(int[] pre, int[] in) {
        if(pre.length == 0 ||in.length == 0)
            return null;
        return ConstructBinaryTree(pre, 0, pre.length-1, in, 0, in.length - 1);
    }
    public TreeNode ConstructBinaryTree(int[] pre, int pstart, int pend, int[] in, int istart, int iend){
        if(pstart > pend || istart > iend)
            return null;
        TreeNode root = new TreeNode(pre[pstart]);
        for(int i = istart; i <= iend; i++){
            if(in[i] == pre[pstart]){
                root.left = ConstructBinaryTree(pre, pstart+1, pstart + i - istart, in, istart, i - 1);
                root.right = ConstructBinaryTree(pre, pstart + i - istart + 1, pend, in, i + 1, iend);
                break;
            }
        }
        return root;
    }
}