JZ-004-重建二叉树

重建二叉树

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。

题目链接: 重建二叉树

import java.util.*;

public class Jz04 {

    // 缓存中序遍历数组每个值对应的索引
    private static Map<Integer, Integer> indexForInOrders = new HashMap<Integer, Integer>();

    public static TreeNode reConstructBinaryTree(int[] pre, int[] in) {
        for (int i = 0; i < in.length; i++) {
            indexForInOrders.put(in[i], i);
        }
        return reConstructBinaryTree(pre, 0, pre.length - 1, 0);
    }

    /**
     * 递归
     *
     * @param pre
     * @param preL
     * @param preR
     * @param inL
     * @return
     */
    private static TreeNode reConstructBinaryTree(int[] pre, int preL, int preR, int inL) {
        if (preL > preR) {
            return null;
        }
        TreeNode root = new TreeNode(pre[preL]);
        int inIndex = indexForInOrders.get(root.val);
        int leftTreeSize = inIndex - inL;
        root.left = reConstructBinaryTree(pre, preL + 1, preL + leftTreeSize, inL);
        root.right = reConstructBinaryTree(pre, preL + leftTreeSize + 1, preR, inL + leftTreeSize + 1);
        return root;
    }

    public static void main(String[] args) {
          // 测试用例，并打印最后结果
        int[] pre = new int[]{3, 9, 20, 15, 7};
        int[] in = new int[]{9, 3, 15, 20, 7};
        TreeNode treeNode = reConstructBinaryTree(pre, in);
        Queue<TreeNode> nodes = new LinkedList<>();
        nodes.add(treeNode);
        while (!nodes.isEmpty()) {
            TreeNode cur = nodes.poll();
            if (cur != null) {
                System.out.println(cur.val + " ");
                nodes.add(cur.left);
                nodes.add(cur.right);
            }
        }
    }
}

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