106. construct binary tree from middle order and post order traversal sequence

One 、 Title Description

Given two arrays of integers inorder and postorder , among inorder Is the middle order traversal of a binary tree , postorder Is the post order traversal of the same tree , Please construct and return this Binary tree .

Example 1：

 Input ：inorder = [9,3,15,20,7], postorder = [9,15,7,20,3]
 Output ：[3,9,20,null,null,15,7]

Example 2：

 Input ：inorder = [-1], postorder = [-1]
 Output ：[-1]

Two 、 Problem solving

Find four nodes

/** * 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 {
    
    Map<Integer,Integer> map = new HashMap<>();
    public TreeNode buildTree(int[] inorder, int[] postorder) {
    
        // This is similar to the construction of binary tree by preorder traversal and inorder traversal 
        // Mainly find four nodes   Use hashMap Save the nodes traversed in the middle order 
        if(inorder == null || postorder == null || inorder.length == 0 || postorder.length == 0){
    
            return null;
        }
        // Save the nodes traversed in the middle order 
        for(int i = 0;i<inorder.length;i++){
    
            map.put(inorder[i],i);
        }
        return myBuildTree(inorder,postorder,0,inorder.length-1,0,postorder.length-1);      
    }
    public TreeNode myBuildTree(int[] inorder, int[] postorder,int ileft,int iright,int pleft,int pright){
    
        // Judge array 
        if(ileft > iright || pleft > pright){
    
            return null;
        }
        // Find the root node 
        int rootnum = postorder[pright];
        // Create a root node 
        TreeNode root = new TreeNode(rootnum);
        // Split array 
        int inorderindex = map.get(rootnum);
        // The length of the left subtree 
        int leftlength = inorderindex - ileft;
        // The next segment of the postorder traversal 
        root.left = myBuildTree(inorder,postorder,ileft,inorderindex-1,pleft,pleft+leftlength-1);
        root.right = myBuildTree(inorder,postorder,inorderindex+1,iright,pleft+leftlength,pright-1);
        return root;
    }
}