第四周 学习总结

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    Stack<TreeNode> stack = new Stack<>();
    TreeNode curr = root ;
    while (curr != null || !stack.isEmpty()){
        while (curr != null){
            stack.push(curr);
            curr = curr.left;
        }
        curr = stack.pop();
        res.add(curr.val);
        curr = curr.right;
    }
    return res;
}
}

public boolean isValidBST(TreeNode root) {
   if (root == null) return true;
   Stack<TreeNode> stack = new Stack<>();
   TreeNode pre = null;
   while (root != null || !stack.isEmpty()) {
      while (root != null) {
         stack.push(root);
         root = root.left;
      }
      root = stack.pop();
      if(pre != null && root.val <= pre.val) return false;
      pre = root;
      root = root.right;
   }
   return true;
}