剑指Offer（四）重建二叉树

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) {
        return build(pre, in, 0, pre.length-1, 0, in.length-1);
    }

    public TreeNode build(int[] pre, int[] in, int pstart, int pend, int istart, int iend){
        if(pstart > pend)   return null;

        int cur = pre[pstart];//二叉树的根节点
        int find = istart;//游标
        while(find <= iend){//在中序遍历序列中寻找根节点
            if(cur == in[find]) break;
            else find++;
        }

        int len = find-istart;//子树高度
        TreeNode res = new TreeNode(cur);//创建子树根节点
        res.left = build(pre, in, pstart+1, pstart+len, istart, find-1);//创建左子树
        res.right = build(pre, in, pstart+len+1, pend,  find+1, iend);//创建右子树
        return res;
    }
}