【数据结构】算法 Construct Binary Tree from Preorder and Inorder Traversal 从前序与中序遍历序列构造二叉树

目录

• Construct Binary Tree from Preorder and Inorder Traversal 从前序与中序遍历序列构造二叉树
• 思路
• Tag

Construct Binary Tree from Preorder and Inorder Traversal 从前序与中序遍历序列构造二叉树

通过preorder 和inorder，构造一棵二叉树

思路

问题不复杂，有了前序和中序，可以恢复一棵二叉树。

前序里面打头的肯定是root

中序里面的root，将inorder 分成2份，左侧为左树，右侧为右树。

需要pos 定位root，然后递归buildtree

/**
 * 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 {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        List<Integer> pre= new ArrayList<>();
        List<Integer> in= new ArrayList<>();
        int i =0;
        while(i<preorder.length){
            pre.add(preorder[i]);
            i++;
        }         
        i=0;
        while(i<inorder.length){
            in.add(inorder[i]);
            i++;
        }
        return _buildTree(pre,in);         
    }

    public TreeNode _buildTree(List<Integer> preorder, List<Integer> inorder){
        if(preorder.size()==0){return null;}
        int pos =0 ;          
        while(!inorder.get(pos).equals(preorder.get(0))){
            pos++;
        }
        List<Integer> lpre = new ArrayList<>();
        List<Integer> lin = new ArrayList<>();
        List<Integer> rpre = new ArrayList<>();
        List<Integer> rin = new ArrayList<>();
        for(int i = 0;i<pos ;i++){
            lpre.add(preorder.get(i+1));
            lin.add(inorder.get(i));
        }
        for(int i = pos+1;i<inorder.size();i++){
            rpre.add(preorder.get(i));
            rin.add(inorder.get(i));
        }
        TreeNode root = new TreeNode(preorder.get(0)) ;
        root.left = _buildTree(lpre,lin);
        root.right = _buildTree(rpre,rin);
        return root;
    }

}

Tag

tree DFS Array