20220508树和图：从前序与中序遍历序列构造二叉树

        题目描述：给定两个整数数组 preorder 和 inorder ，其中 preorder 是二叉树的先序遍历， inorder 是同一棵树的中序遍历，请构造二叉树并返回其根节点。

        编码实现：

public TreeNode buildTree(int[] preorder, int[] inorder) {
        if (null == preorder || preorder.length == 0){
            return null;
        }
        Stack s = new Stack<>();
        TreeNode root = new TreeNode(preorder[0]);
        TreeNode cur = root;
        for (int i = 1, j = 0; i < preorder.length; i++) {
            if (cur.val != inorder[j]) {
                cur.left = new TreeNode(preorder[i]);
                s.push(cur);
                cur = cur.left;
            } else {
                j++;
                while (!s.empty() && s.peek().val == inorder[j]) {
                    cur = s.pop();
                    j++;
                }
                cur = cur.right = new TreeNode(preorder[i]);
            }
        }
        return root;
    }

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;
      }
}