889. Construct Binary Tree from Preorder and Postorder Traversal - Medium

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode constructFromPrePost(int[] pre, int[] post) {
        return buildTree(pre, 0, pre.length - 1, post, 0, post.length - 1);
    }
    
    public TreeNode buildTree(int[] pre, int pre_start, int pre_end, int[] post, int post_start, int post_end) {
        if(pre_start > pre_end || post_start > post_end) {
            return null;
        }
        
        TreeNode root = new TreeNode(pre[pre_start]);
        if(pre_start + 1 > pre_end) {
            return root;
        }
        
        int idx = 0;
        for(int i = post_start; i <= post_end; i++) {
            if(post[i] == pre[pre_start + 1]) {
                idx = i;
                break;
            }
        }
        
        root.left = buildTree(pre, pre_start + 1, pre_start + idx - post_start + 1, post, post_start, idx);
        root.right = buildTree(pre, pre_start + idx - post_start + 2, pre_end, post, idx + 1, post_end - 1);
        
        return root;
    }
}