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http://blog.csdn.net/xiaofei_it/article/details/51502254

为了防止思维僵化，每天刷个算法题。已经刷了几天了，现在贴点代码。

2002年我初中二年级，开始学习BASIC语言。2004年中考之后，开始参加NOIP，系统学习算法。一直非常喜欢算法，但工作后几乎不再碰这些东西。现在准备重新捡起来。

我已经建了一个开源项目，每天的题目都在里面：

https://github.com/Xiaofei-it/Algorithms

绝大部分算法都是我自己写的，没有参考网上通用代码。读者可能会觉得有的代码晦涩难懂，因为那是我自己的理解。

最近几天都是在写一些原来的东西，大多数是非递归。以后准备刷点DP、贪心之类的题。

这里贴一个递归与非递归二叉树遍历。

前序遍历

/**
 *
 * Copyright 2016 Xiaofei
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

package xiaofei.algorithm;

/**
 * Created by Xiaofei on 16/5/18.
 */

public class PreOrderTraversal {
    public static void traversalRecursively(BinaryTreeNode root) {
        if (root == null) {
            return;
        }
        System.out.print(root.data + " ");
        traversalRecursively(root.left);
        traversalRecursively(root.right);
    }

    public static void traversalCorecursively(BinaryTreeNode root) {
        class StackElement {
            BinaryTreeNode node;
            boolean leftVisited;
            boolean rightVisited;
        }
        StackElement[] stack = new StackElement[100];
        for (int i = 0; i < 100; ++i) {
            stack[i] = new StackElement();
        }
        int top = -1;
        stack[++top] = new StackElement();
        stack[top].node = root;
        while (top >= 0) {
            BinaryTreeNode node = stack[top].node;
            if (!stack[top].leftVisited) {
                System.out.print(node.data + " ");
                stack[top].leftVisited = true;
                if (node.left != null) {
                    stack[++top] = new StackElement();
                    stack[top].node = node.left;
                }
            } else if (!stack[top].rightVisited) {
                stack[top].rightVisited = true;
                if (node.right != null) {
                    stack[++top] = new StackElement();
                    stack[top].node = node.right;
                }
            } else {
                --top;
            }
        }

    }

}

中序遍历

/**
 *
 * Copyright 2016 Xiaofei
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

package xiaofei.algorithm;

/**
 * Created by Xiaofei on 16/5/18.
 */

public class InOrderTraversal {
    public static void traversalRecursively(BinaryTreeNode root) {
        if (root == null) {
            return;
        }
        traversalRecursively(root.left);
        System.out.print(root.data + " ");
        traversalRecursively(root.right);
    }

    public static void traversalCorecursively(BinaryTreeNode root) {
        class StackElement {
            BinaryTreeNode node;
            boolean leftVisited;
            boolean rightVisited;
        }
        StackElement[] stack = new StackElement[100];
        for (int i = 0; i < 100; ++i) {
            stack[i] = new StackElement();
        }
        int top = -1;
        stack[++top] = new StackElement();
        stack[top].node = root;
        while (top >= 0) {
            BinaryTreeNode node = stack[top].node;
            if (!stack[top].leftVisited) {
                stack[top].leftVisited = true;
                if (node.left != null) {
                    stack[++top] = new StackElement();
                    stack[top].node = node.left;
                }
            } else if (!stack[top].rightVisited) {
                System.out.print(node.data + " ");
                stack[top].rightVisited = true;
                if (node.right != null) {
                    stack[++top] = new StackElement();
                    stack[top].node = node.right;
                }
            } else {
                --top;
            }
        }

    }

}

后序遍历

/**
 *
 * Copyright 2016 Xiaofei
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

package xiaofei.algorithm;

/**
 * Created by Xiaofei on 16/5/18.
 */

public class PostOrderTraversal {
    public static void traversalRecursively(BinaryTreeNode root) {
        if (root == null) {
            return;
        }
        traversalRecursively(root.left);
        traversalRecursively(root.right);
        System.out.print(root.data + " ");
    }

    public static void traversalCorecursively(BinaryTreeNode root) {
        class StackElement {
            BinaryTreeNode node;
            boolean leftVisited;
            boolean rightVisited;
        }
        StackElement[] stack = new StackElement[100];
        for (int i = 0; i < 100; ++i) {
            stack[i] = new StackElement();
        }
        int top = -1;
        stack[++top] = new StackElement();
        stack[top].node = root;
        while (top >= 0) {
            BinaryTreeNode node = stack[top].node;
            if (!stack[top].leftVisited) {
                stack[top].leftVisited = true;
                if (node.left != null) {
                    stack[++top] = new StackElement();
                    stack[top].node = node.left;
                }
            } else if (!stack[top].rightVisited) {
                stack[top].rightVisited = true;
                if (node.right != null) {
                    stack[++top] = new StackElement();
                    stack[top].node = node.right;
                }
            } else {
                System.out.print(node.data + " ");
                --top;
            }
        }

    }

}