二叉樹遍歷（前序、中序、後序）-Java實現

 

1、前序遍歷

　　訪問順序：先根節點，再左子樹，最後右子樹；上圖的訪問結果爲：GDAFEMHZ。

　　1）遞歸實現

public void preOrderTraverse1(TreeNode root) { if (root != null) { System.out.print(root.val + "->"); preOrderTraverse1(root.left); preOrderTraverse1(root.right); } }

　　2）非遞歸實現

public void preOrderTraverse2(TreeNode root) { Stack<TreeNode> stack = new Stack<>(); TreeNode node = root; while (node != null || !stack.empty()) { if (node != null) { System.out.print(node.val + "->"); stack.push(node); node = node.left; } else { TreeNode tem = stack.pop(); node = tem.right; } } }

2、中序遍歷

　　訪問順序：先左子樹，再根節點，最後右子樹；上圖的訪問結果爲：ADEFGHMZ。

　　1）遞歸實現

public void inOrderTraverse(TreeNode root) { if (root != null) { inOrderTraverse(root.left); System.out.print(root.val + "->"); inOrderTraverse(root.right); } }

　　2）非遞歸實現

public void inOrderTraverse(TreeNode root) { Stack<TreeNode> stack = new Stack<>(); TreeNode node = root; while (node != null || !stack.isEmpty()) { if (node != null) { stack.push(node); node = node.left; } else { TreeNode tem = stack.pop(); System.out.print(tem.val + "->"); node = tem.right; } } }

3、後序遍歷

　　訪問順序：先左子樹，再右子樹，最後根節點，上圖的訪問結果爲：AEFDHZMG。

　　1）遞歸實現

public void postOrderTraverse(TreeNode root) { if (root != null) { postOrderTraverse(root.left); postOrderTraverse(root.right); System.out.print(root.val + "->"); } }

 　　2）非遞歸實現

public void postOrderTraverse(TreeNode root) { TreeNode cur, pre = null; Stack<TreeNode> stack = new Stack<>(); stack.push(root); while (!stack.empty()) { cur = stack.peek(); if ((cur.left == null && cur.right == null) || (pre != null && (pre == cur.left || pre == cur.right))) { System.out.print(cur.val + "->"); stack.pop(); pre = cur; } else { if (cur.right != null) stack.push(cur.right); if (cur.left != null) stack.push(cur.left); } } }

 4、層次遍歷

　　訪問結果：GDMAFHZE。

public void levelOrderTraverse(TreeNode root) { if (root == null) { return; } Queue<TreeNode> queue = new LinkedList<TreeNode>(); queue.add(root); while (!queue.isEmpty()) { TreeNode node = queue.poll(); System.out.print(node.val + "->"); if (node.left != null) { queue.add(node.left); } if (node.right != null) { queue.add(node.right); } } }