深度优先搜索：从起始节点开始，沿着一条路径尽可能深地探索，直到无法继续，然后回溯到上一个节点，再选择另一条路径继续深入探索，以此类推，直到访问完所有可达节点。

/**
 * Definition for a binary tree node.
 * public 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;
 *     }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return root;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            // 交换左右子树
            TreeNode temp = node.left;
            node.left = node.right;
            node.right = temp;
            
            if (node.right != null) stack.push(node.right);
            if (node.left != null) stack.push(node.left);
        }
        return root;
    }
}

递归实现深度优先搜索：

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root ==null){
            return root;
        }

        TreeNode left = invertTree(root.left);
        TreeNode right = invertTree(root.right);
        root.left = right;
        root.right = left;
        return root;
    }
}

广度优先搜索：从起始节点开始，逐层地对节点进行访问，先访问起始节点的所有邻接节点，然后依次访问这些邻接节点的邻接节点，按照距离起始节点由近到远的顺序进行遍历。

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null){
            return root;
        }
        Queue<TreeNode> quu = new LinkedList<>();

        quu.offer(root);
        
        while(!quu.isEmpty()){
            //int size = queue.size()，因为只是翻转，不需要过于在意是哪一行
            TreeNode node = quu.poll();
            if(node.left != null) quu.offer(node.left);
            if(node.right != null) quu.offer(node.right);
            TreeNode temp = node.left;
            node.left = node.right;
            node.right = temp;
        }
        return root;
    }
}