1、定义树

可以参考链表，链表遍历不方便，如果单链表有多个next指针，则就形成了树。

Java:

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

Python：

class TreeNode(object):def __init__(self, val=0, left=None, right=None):self.val = valself.left, self.right = None, None

C++：

struct TreeNode {int val;TreeNode *left;TreeNode *right;TreeNode(int x) : val(x), left(null), right(null) {}
};

C:

struct TreeNode {int val;struct TreeNode *left;struct TreeNode *right;
};

 2、树的遍历

• 前序遍历：根节点→左子树→右子树

• 中序遍历：左子树→根节点→右子树

• 后序遍历：左子树→右子树→根节点

• BFS（广度优先搜索）：先访问上一层，在访问下一层，一层一层的往下访问

• DFS（深度优先搜索）：先访根节点，然后左结点，一直往下，直到最左结点没有子节点的时候然后往上退一步到父节点，然后父节点的右子节点在重复上面步骤……

代码：

1）Python前序、中序、后序遍历：

def preorder(self, root):if root:self.traverse_path.append(root.val);self.preorder(root.left);self.preorder(root.right);def inorder(self, root):if root:self.preorder(root.left);self.traverse_path.append(root.val);self.preorder(root.right);def postorder(self, root):if root:self.preorder(root.left);self.preorder(root.right);self.traverse_path.append(root.val);

2）Java遍历：

前序遍历：

// 递归
public static void preOrder(TreeNode tree) {if (tree == null)return;System.out.printf(tree.val + "");preOrder(tree.left);preOrder(tree.right);
}
// 非递归
public static void preOrder(TreeNode tree) {if (tree == null) return;Stack<TreeNode> q1 = new Stack<>();q1.push(tree);//压栈while (!q1.empty()) {TreeNode t1 = q1.pop();//出栈System.out.println(t1.val);if (t1.right != null) {q1.push(t1.right);}if (t1.left != null) {q1.push(t1.left);}}
}

中序遍历：

// 递归
public static void inOrder(TreeNode node) {if (node == null)  return;inOrder(node.left);System.out.println(node.val);inOrder(node.right);
}
// 非递归
public static void inOrder(TreeNode tree) {Stack<TreeNode> stack = new Stack<>();while (tree != null || !stack.isEmpty()) {while (tree != null) {stack.push(tree);tree = tree.left;}if (!stack.isEmpty()) {tree = stack.pop();System.out.println(tree.val);tree = tree.right;}}
}

后序遍历：

// 递归
public static void postOrder(TreeNode tree) {if (tree == null) return;postOrder(tree.left);postOrder(tree.right);System.out.println(tree.val);
}
// 非递归
public static void postOrder(TreeNode tree) {if (tree == null)return;Stack<TreeNode> s1 = new Stack<>();Stack<TreeNode> s2 = new Stack<>();s1.push(tree);while (!s1.isEmpty()) {tree = s1.pop();s2.push(tree);if (tree.left != null) {s1.push(tree.left);}if (tree.right != null) {s1.push(tree.right);}}while (!s2.isEmpty()) {System.out.print(s2.pop().val + " ");}
}public static void postOrder(TreeNode tree) {if (tree == null)return;Stack<TreeNode> stack = new Stack<>();stack.push(tree);TreeNode c;while (!stack.isEmpty()) {c = stack.peek();if (c.left != null && tree != c.left && tree != c.right) {stack.push(c.left);} else if (c.right != null && tree != c.right) {stack.push(c.right);} else {System.out.print(stack.pop().val + " ");tree = c;}}
}

BFS（广度优先搜索）：

// 递归
public static void levelOrder(TreeNode tree) {int depth = depth(tree);for (int level = 0; level < depth; level++) {printLevel(tree, level);}
}private static int depth(TreeNode tree) {if (tree == null) return 0;int leftDepth = depth(tree.left);int rightDepth = depth(tree.right);return Math.max(leftDepth, rightDepth) + 1;
}private static void printLevel(TreeNode tree, int level) {if (tree == null) return;if (level == 0) {System.out.print(" " + tree.val);} else {printLevel(tree.left, level - 1);printLevel(tree.right, level - 1);}
}// 非递归
public static void levelOrder1(TreeNode tree) {if (tree == null) return;LinkedList<TreeNode> list = new LinkedList<>(); // 链表，可以把它看做队列list.add(tree); // 相当于把数据加入到队列尾部while (!list.isEmpty()) {TreeNode node = list.poll(); // poll方法相当于移除队列头部的元素System.out.println(node.val);if (node.left != null) list.add(node.left);if (node.right != null) list.add(node.right);}
}// 结果存放到list中
public static List<List<Integer>> levelOrder2(TreeNode tree) {if (tree == null)return null;List<List<Integer>> list = new ArrayList<>();bfs(tree, 0, list);return list;
}private static void bfs(TreeNode tree, int level, List<List<Integer>> list) {if (tree == null)return;if (level >= list.size()) {List<Integer> subList = new ArrayList<>();subList.add(tree.val);list.add(subList);} else {list.get(level).add(tree.val);}bfs(tree.left, level + 1, list);bfs(tree.right, level + 1, list);
}

DFS（深度优先搜索）：

// 递归
public static void treeDFS(TreeNode root) {if (root == null) return;System.out.println(root.val);treeDFS(root.left);treeDFS(root.right);
}// 非递归
public static void treeDFS1(TreeNode root) {Stack<TreeNode> stack = new Stack<>();stack.add(root);while (!stack.empty()) {TreeNode node = stack.pop();System.out.println(node.val);if (node.right != null) {stack.push(node.right);}if (node.left != null) {stack.push(node.left);}}
}