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。

没啥说的递归建树 很方便的~

/**
 * 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) {
java.util.HashMap<Integer,Integer> map = new java.util.HashMap<>();
for (int i = 0; i < in.length; i++) {
map.put(in[i], i);
}
return reConstructBinaryTree(0, pre.length-1, pre, in, 0, in.length-1, map);
    }


public TreeNode reConstructBinaryTree(int i, int j, int[] pre, int[] in, int s, int e, java.util.HashMap<Integer, Integer> map) {
// TODO Auto-generated method stub
if (i > j) return null;
TreeNode node = new TreeNode(pre[i]);
int index = map.get(pre[i]);
if (i == j) return node;
node.left = reConstructBinaryTree(i+1, i+index-s, pre, in, s, index-1, map);
node.right = reConstructBinaryTree(i+index-s+1, j, pre, in, index+1, e, map);
return node;
}
}