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 {
    int now;
    public void work(int []index, int []pre, int l, int r, TreeNode p) {
        p.val = pre[now++];
        
        if (now < pre.length && index[pre[now]] >= l && index[pre[now]] <= r && 
            index[pre[now]] < index[p.val]) {
            p.left = new TreeNode(0);
            work(index, pre,  l, index[p.val] - 1, p.left);
            
        } 
        if (now < pre.length && index[pre[now]] >= l && index[pre[now]] <= r && 
                   index[pre[now]] > index[p.val]) {
            p.right = new TreeNode(0);
            work(index, pre, index[p.val] + 1, r, p.right);
        }
    }
    public TreeNode reConstructBinaryTree(int [] pre,int [] in) {
        now = 0;
        int Max = 0;
        for (int i = 0; i < pre.length; ++i) {
            Max = Math.max(pre[i], Max);
        }
        int []index = new int [Max + 1];
        for (int i = 0; i < in.length; ++i) {
            index[ in[i] ]  = i;
        }
        TreeNode p = new TreeNode(0);
        work(index, pre, 0, pre.length - 1, p);
        return p;
    }
}