leetcode144_145_94_二叉树的遍历

这是根据代码随想录的刷题顺序进行的刷题：https://programmercarl.com/二叉树的递归遍历.html

递归版

递归中最重要的事儿：

1. 参数的意义与类型
2. 结束条件
3. 递归体

144 前序遍历

public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> list = new ArrayList<Integer>();
    f(list,root);
    return list;
}
private void f(List<Integer> list, TreeNode root) {
    if(root == null) return;
    list.add(root.val);
    f(list, root.left);
    f(list, root.right);
}

145 后序遍历

public List<Integer> postorderTraversal(TreeNode root) {
    List<Integer> list = new ArrayList<Integer>();
    f(list, root);
    return list;
}
private void f(List<Integer> list, TreeNode root) {
    if(root == null) return;
    f(list, root.left);
    f(list, root.right);
    list.add(root.val);
}

94 中序遍历

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        f(list, root);
        return list;
    }
    private void f(List<Integer> list, TreeNode root) {
        if(root == null) return;
        f(list, root.left);
        list.add(root.val);
        f(list, root.right);
    }
}

迭代

144前序遍历

//前序遍历：中-左-右 入栈顺序：中-右-左
public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> list = new ArrayList<Integer>();
    if(root == null) return list;
    Stack<TreeNode> stack = new Stack<TreeNode>();
    stack.push(root);
    while(!stack.isEmpty()) {
        TreeNode node = stack.pop();
        list.add(node.val);
        if(node.right!= null) {
            stack.push(node.right);
        }
        if(node.left != null) {
            stack.push(node.left);
        }
    }
    return list;
}

145后序遍历

后序遍历需要的是左右中，左右中可以由中右左得到，而前序遍历是中左右，只需要再前序遍历的基础上修改一下顺序并再最后反转一下结果即可得到后序遍历的结果。

public List<Integer> postorderTraversal(TreeNode root) {
    List<Integer> list = new ArrayList<Integer>();
    if(root == null) return list;
    Stack<TreeNode> stack = new Stack();
    stack.push(root);
    while(!stack.isEmpty()) {
        TreeNode node = stack.pop();
        list.add(node.val);
        if(node.left != null) stack.push(node.left);
        if(node.right!= null) stack.push(node.right);
    }
    Collections.reverse(list);
    return list;
}

94中序遍历

public List<Integer> inorderTraversal(TreeNode root) {
    Stack<TreeNode> stack = new Stack();
    List<Integer> list = new ArrayList<Integer>();
    TreeNode cur = root;
    while(cur != null || !stack.isEmpty()) {
        if(cur != null) {
            stack.push(cur);
            cur = cur.left;
        }
        else {
            cur = stack.pop();
            list.add(cur.val);
            cur = cur.right;
        }
    }
    return list;
}