Binary Tree Postorder Traversal

参考链接1  参考链接2

两个stack：

public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> s1 = new Stack<TreeNode>();
        Stack<TreeNode> s2 = new Stack<TreeNode>();
        List<Integer> result = new ArrayList<Integer>();
        if(root == null) return result; 
        s1.push(root);
        while(!s1.isEmpty()) {
            TreeNode pop = s1.pop();
            s2.push(pop);
            if(pop.left != null) s1.push(pop.left);
            if(pop.right != null) s1.push(pop.right);
        }
        while(!s2.isEmpty()) {
            result.add(s2.pop().val);
        }
        return result;
    }
}

一个Stack：

public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> s = new Stack<TreeNode>();
        List<Integer> result = new ArrayList<Integer>();
        if(root == null) return result;
        s.push(root);
        TreeNode prev = null;
        while(!s.isEmpty()) {
            TreeNode curr = s.peek();
            if(prev == null || prev.left == curr || prev.right == curr) {
                if(curr.left != null) {
                    s.push(curr.left);
                } else if(curr.right != null) {
                    s.push(curr.right);
                } else {
                    result.add(curr.val);
                    s.pop();
                }
            } else if(curr.left == prev) {
                if(curr.right != null) {
                    s.push(curr.right);
                } else {
                    result.add(curr.val);
                    s.pop();
                }
            } else/* if(curr.right == prev)*/ {
                result.add(curr.val);
                s.pop();
            }
            prev = curr;
        }
        return result;
    }
}