144. Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:

Given binary tree

{1,#,2,3}

,

1
\
2
/
3

return

[1,2,3]

.

思路：普通递归的先序遍历就就不写了，下面给出一种非递归的先序遍历，用stack来实现

public static void preorder(List<Integer> list, TreeNode root) {

if (root == null)

return;

Stack<TreeNode> stack = new Stack<TreeNode>();

while (root != null || !stack.isEmpty()) {

while (root != null) {

list.add(root.val);

stack.push(root);

root = root.left;

}

root=stack.pop();

root=root.right;

}

}