lintcode66-67-68 Binary Tree (Postorder-Inorder-Postorder) Traversal 非递归(iterative)总结

Given a binary tree, return the preoder, inorder, postorder traversal of its nodes' values.

 

  模板对

	前序	中序	后序
全局变量定义	ArrayList<Integer> result + Stack<TreeNode> stack 好基友
	TreeNode curt	TreeNode curt	TreeNode prev，curt
判断输入	都判root == null ?，最后看能不能合并，inorder其实可以
初始化	push root	curt = root	push root, curt = root, prev = null
while条件	!stack.isEmpty()	curt != null || !stack.isEmpty()	!stack.isEmpty()
while内部结构	add中, 两个独立if去push右左	while一直左下, add中, 赋值以处理右	curt = peek; 三个关联if处理下溯，左上溯，右上溯或叶子{ 下溯，两个关联if去push左右 左上溯，push右 右上溯，add } prev = curt;
return

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1.preorder

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        List<Integer> preorder = new ArrayList<Integer>();
        
        if (root == null) {
            return preorder;
        }
        
        stack.push(root);
        while (!stack.empty()) {
            TreeNode node = stack.pop();
            preorder.add(node.val);
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        
        return preorder;
    }
}

 

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2.inorder

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        ArrayList<Integer> result = new ArrayList<Integer>();
        TreeNode curt = root;
        while (curt != null || !stack.empty()) {
            while (curt != null) {
                stack.add(curt);
                curt = curt.left;
            }
            curt = stack.pop();
            result.add(curt.val);
            curt = curt.right;
        }
        return result;
    }
}

 

 

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3. postorder

public ArrayList<Integer> postorderTraversal(TreeNode root) {
    ArrayList<Integer> result = new ArrayList<Integer>();
    Stack<TreeNode> stack = new Stack<TreeNode>();
    TreeNode prev = null; // previously traversed node
    TreeNode curr = root;

    if (root == null) {
        return result;
    }

    stack.push(root);
    while (!stack.empty()) {
        curr = stack.peek();
        if (prev == null || prev.left == curr || prev.right == curr) { // traverse down the tree
            if (curr.left != null) {
                stack.push(curr.left);
            } else if (curr.right != null) {
                stack.push(curr.right);
            }
        } else if (curr.left == prev) { // traverse up the tree from the left
            if (curr.right != null) {
                stack.push(curr.right);
            }
        } else { // traverse up the tree from the right
            result.add(curr.val);
            stack.pop();
        }
        prev = curr;
    }

    return result;
}