LeetCode剑指 Offer 07. 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。链接

思路

map记录中序遍历节点的下标，方便后续前序通过根节点定位中序遍历的位置。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    
    Map<Integer, Integer> map = new HashMap<>();
    int[] preorder;
    int[] inorder;
    public TreeNode buildTree(int[] preorder, int[] inorder) {
    
    
        for(int i = 0; i < inorder.length; i++){
    
    
            map.put(inorder[i], i);
        }
        this.preorder = preorder;
        this.inorder = inorder;
        return buildTree(0, 0, inorder.length - 1);
    }

    TreeNode buildTree(int preIdx, int inLeft, int inRight){
    
    
        if(inLeft > inRight)return null;
        TreeNode root = new TreeNode(preorder[preIdx]);
        int inRootIdx = map.get(preorder[preIdx]);
        //preIdx的下一个即为左子树根节点
        root.left = buildTree(preIdx + 1, inLeft, inRootIdx - 1);
        //右子树根节点需要跳过所有左子树的节点，左子树节点一共有inRootIdx - inLeft个
        //在去除左子树的根节点preIdx+(1+inRootIdx-inLeft)
        root.right = buildTree(preIdx + 1 + inRootIdx - inLeft, inRootIdx + 1, inRight);
        return root;
    }
}