給定一個二叉樹，返回它的 後序遍歷。

來源：力扣（LeetCode）
連結：https://leetcode-cn.com/problems/binary-tree-postorder-traversal
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遞迴

import java.util.ArrayList;
import java.util.List;

class Solution {

    private List<Integer> ret = new ArrayList<>();

    private void solve(TreeNode root) {
        if (root == null) {
            return;
        }
        solve(root.left);
        solve(root.right);
        ret.add(root.val);
    }

    public List<Integer> postorderTraversal(TreeNode root) {
        solve(root);
        return ret;
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

 

棧

import java.util.*;

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }

        Stack<TreeNode> stack = new Stack<>();
        TreeNode prev = null;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }

            root = stack.pop();

            if (root.right == null || root.right == prev) {
                res.add(root.val);
                prev = root;
                root = null;
            } else {
                stack.push(root);
                root = root.right;
            }
        }

        return res;
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

 

Morris遍歷

import java.util.ArrayList;
import java.util.List;

class Solution {

    private List<Integer> ret = new ArrayList<>();

    private TreeNode reverse(TreeNode root) {
        TreeNode pre = null, cur = root, next;
        while (cur != null) {
            next = cur.right;
            cur.right = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }

    private void getRightEdge(TreeNode root) {
        TreeNode head = reverse(root);
        TreeNode cur = head;
        while (cur != null) {
            ret.add(cur.val);
            cur = cur.right;
        }
        reverse(head);
    }


    public List<Integer> postorderTraversal(TreeNode root) {
        TreeNode cur = root;

        while (cur != null) {
            TreeNode mostRight = cur.left;
            if (mostRight != null) {
                while (mostRight.right != null && mostRight.right != cur) {
                    mostRight = mostRight.right;
                }
                if (mostRight.right == null) {
                    mostRight.right = cur;
                    cur = cur.left;
                    continue;
                } else {
                    mostRight.right = null;
                    getRightEdge(cur.left);
                }
            }
            cur = cur.right;
        }
        getRightEdge(root);
        return ret;
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}