Construct Binary Tree from Inorder and Postorder Traversal

题目
Given inorder and postorder traversal of a tree, construct the binary tree.

答案

class Solution {
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        return build(inorder, postorder, 0, postorder.length - 1, 0, inorder.length - 1);   
    }
    
    public TreeNode build(int[] inorder, int[] postorder, int pl, int pr, int il, int ir) {
        if(pl > pr) return null;
        // First, take postorder[pr] as root
        TreeNode root = new TreeNode(postorder[pr]);
        
        // Second, find root in array inorder
        int root_idx = il;
        for(int i = il; i <= ir; i++) {
            if(inorder[i] == root.val) root_idx = i;
        }
        int left_size = root_idx - il;
        int right_size = ir - root_idx;
        // Third, build subrees
        root.left = build(inorder, postorder, pr - right_size - left_size, pr - right_size - 1, il, root_idx - 1);
        root.right = build(inorder, postorder, pr - right_size, pr - 1, root_idx + 1, ir);
        return root;

    }
}