后序遍历

一些大神的总结

http://blog.sina.com.cn/s/blog_eb52001d0102v1si.html

递归的两种，写在一个方法里或者两个

Recursive

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Postorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> result = new ArrayList<Integer>();
        traverse(root, result);
        return result;
    }
    private void traverse(TreeNode root, ArrayList<Integer> result) {
        if (root == null) {
            return;
        }
        traverse(root.left, result);
        traverse(root.right, result);
        result.add(root.val);
    }
}

public ArrayList<Integer> postorderTraversal(TreeNode root) {
    ArrayList<Integer> result = new ArrayList<Integer>();

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

    result.addAll(postorderTraversal(root.left));
    result.addAll(postorderTraversal(root.right));
    result.add(root.val);

    return result;   
}

分治

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Postorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> result = new ArrayList<Integer>();
        if (root == null) {
            return result;//不能只写return，注意要求返回类型
        }
        //divide
        ArrayList<Integer> left = postorderTraversal(root.left);
        ArrayList<Integer> right = postorderTraversal(root.right);
        //conquer
        result.addAll(left);
        result.addAll(right);
        result.add(root.val);//又忘了！！！！root.val not root!!
        return result;
    }
}

非递归

//Iterative
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;
}