Java实现 LeetCode 106 从中序与后序遍历序列构造二叉树

106. 从中序与后序遍历序列构造二叉树

根据一棵树的中序遍历与后序遍历构造二叉树。

注意:
你可以假设树中没有重复的元素。

例如，给出

中序遍历 inorder = [9,3,15,20,7]
后序遍历 postorder = [9,15,7,20,3]
返回如下的二叉树：

    3
   / 
  9  20
    /  
   15   7

 
class Solution {
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        return helper(inorder, postorder, postorder.length - 1, 0, inorder.length - 1);
    }

    public TreeNode helper(int[] inorder, int[] postorder, int postEnd, int inStart, int inEnd) {
        if (inStart > inEnd) {
            return null;
        }

        int currentVal = postorder[postEnd];
        TreeNode current = new TreeNode(currentVal);
        
        int inIndex = 0; 
        for (int i = inStart; i <= inEnd; i++) {
            if (inorder[i] == currentVal) {
                inIndex = i;
            }
        }
        TreeNode left = helper(inorder, postorder, postEnd - (inEnd- inIndex) - 1, inStart, inIndex - 1);
        TreeNode right = helper(inorder, postorder, postEnd - 1, inIndex + 1, inEnd);
        current.left = left;
        current.right = right;
        return current;
    }
}