day46（12.27）——leetcode面试经典150

105. 从前序与中序遍历序列构造二叉树

105. 从前序与中序遍历序列构造二叉树

题目：

题解：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    //创建Map映射
    public int[] preorder; 
    public Map<Integer, Integer> map = new HashMap<>();

    public TreeNode buildTree(int[] preorder, int[] inorder) {
        //为全局变量preorder赋值
        this.preorder = preorder;
        for(int i=0;i<inorder.length;i++) {
            map.put(inorder[i], i);
        }
        return recur(0,0,inorder.length-1);
    }

    TreeNode recur(int root, int left, int right) {
        if(left > right){
            return null;
        }
        //创建结点
        TreeNode node = new TreeNode(preorder[root]);
        //进行获取中间结点
        int i = map.get(preorder[root]);
        node.left = recur(root+1, left, i-1);
        node.right = recur(root+i-left+1, i+1, right);
        return node;
    }
}