[数据结构与算法]二叉树前中后序遍历 递归写法

二叉树前中后序遍历 迭代写法（Java）

前序遍历

Deque<TreeNode> stack = new LinkedList<>();
TreeNode node = root;
while (node != null || !stack.isEmpty()) {
    while (node != null) {
       	ans.add(node.val);
        stack.push(node);
        node = node.left;
    }
    node = stack.pop();
    node = node.right;
}

Deque<TreeNode> stack = new LinkedList<>();
stack.push(root);
while(!stack.isEmpty()){
    TreeNode node=stack.pop();
    ans.add(node.val);
    if(node.right!=null)
        stack.push(node.right);
    if(node.left!=null)
        stack.push(node.left);
}
return ans;

中序遍历

Deque<TreeNode> stack = new LinkedList<>();
TreeNode node = root;
while (node != null || !stack.isEmpty()) {
    while (node != null) {
        stack.push(node);
        node = node.left;
    }
    node = stack.pop();
    ans.add(node.val);
    node = node.right;
}

后序遍历

Deque<TreeNode> stack = new LinkedList<>();
stack.push(root);
while(!stack.isEmpty()){
    TreeNode node=stack.pop();
    ans.addFirst(node.val);
    if(node.left != null)
        stack.push(node.left);
    if(node.right != null)
        stack.push(node.right);
}

TreeNode prev = null;
TreeNode node = root;
while (node != null || !stack.isEmpty()) {
    while (node != null) {
        stack.push(node);
        node = node.left;
    }
    node = stack.pop();
    if (node.right == null || node.right == prev) {
        ans.add(node.val);
        prev = node;
        node = null;
    } else {
        stack.push(node);
        node = node.right;
    }
}

链表数据结构定义

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;
    }
}

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