剑指Offer Java版 面试题7：重建二叉树

题目：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。

练习地址

https://www.nowcoder.com/practice/8a19cbe657394eeaac2f6ea9b0f6fcf6

参考答案

/**
 * 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 == null || pre.length == 0 || in == null || in.length == 0 || pre.length != in.length) {
            return null;
        }
        return construct(pre, in, 0, pre.length - 1, 0, in.length - 1);
    }
    
    private TreeNode construct(int[] pre, int[] in, int preStart, int preEnd, int inStart, int inEnd) {
        if (preStart > preEnd || inStart > inEnd) {
            return null;
        }
        TreeNode node = new TreeNode(pre[preStart]);
        int inMid = inStart;
        while (in[inMid] != pre[preStart]) {
            inMid++;
        }
        int delta = inMid - inStart;
        node.left = construct(pre, in, preStart + 1, preStart + delta, inStart, inMid - 1);
        node.right = construct(pre, in, preStart + delta + 1, preEnd, inMid + 1, inEnd);
        return node;
    }
}

复杂度分析

• 时间复杂度：O(nlogn)。
• 空间复杂度：O(n)。

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