算法——树查找算法

树查找

对于层次结构的树，需要遍历其节点，根据遍历方式不同，可分为广度优先和深度优先，对于如下树结构

class TreeNode<T> {

    T value;
    TreeNode<T> leftChild;
    TreeNode<T> rightChild;

    TreeNode(T value) {
        this.value = value;
    }
}

广度优先搜索

从根节点开始判断，若元素不相等，则将其左右节点存进队列，利用队列先进先出的性质从上往下从左往右依次遍历节点

public <T> boolean BreadthFirstSearch(TreeNode<T> root, T value) {
    Queue<TreeNode<T>> queue = new LinkedList<>();
    StringBuilder stringBuilder = new StringBuilder();
    queue.offer(root);
    while (!queue.isEmpty()) {
        TreeNode<T> node = queue.poll();
        stringBuilder.append(node.value + " ");
        if (node.value == value) {
            System.out.println(stringBuilder.toString() + "found");
            return true;
        }
        if (node.leftChild != null) {
            queue.offer(node.leftChild);
        }
        if (node.rightChild != null) {
            queue.offer(node.rightChild);
        }
    }
    System.out.println(stringBuilder.toString() + "not found");
    return false;
}

调用过程

TreeNode<Integer> node1 = new TreeNode<>(1);
TreeNode<Integer> node2 = new TreeNode<>(2);
TreeNode<Integer> node3 = new TreeNode<>(3);
TreeNode<Integer> node4 = new TreeNode<>(4);
TreeNode<Integer> node5 = new TreeNode<>(5);
node1.leftChild = node2;
node1.rightChild = node3;
node2.leftChild = node4;
node2.rightChild = node5;

BreadthFirstSearch(node1, 4);

深度优先搜索

从根节点开始判断，若元素不相等，则将其左右节点存进栈，利用栈先进后出的性质从上往下依次遍历节点

public <T> boolean DepthFirstSearch(TreeNode<T> root, T value) {
    Stack<TreeNode<T>> stack = new Stack<>();
    StringBuilder stringBuilder = new StringBuilder();
    stack.push(root);
    while (!stack.isEmpty()) {
        TreeNode<T> node = stack.pop();
        stringBuilder.append(node.value + " ");
        if (node.value == value) {
            System.out.println(stringBuilder.toString() + "found");
            return true;
        }
        if (node.rightChild != null) {
            stack.push(node.rightChild);
        }
        if (node.leftChild != null) {
            stack.push(node.leftChild);
        }
    }
    System.out.println(stringBuilder.toString() + "not found");
    return false;
}

• 若右节点先入栈，则先遍历左子树
• 若左节点先入栈，则先遍历右子树

调用过程

TreeNode<Integer> node1 = new TreeNode<>(1);
TreeNode<Integer> node2 = new TreeNode<>(2);
TreeNode<Integer> node3 = new TreeNode<>(3);
TreeNode<Integer> node4 = new TreeNode<>(4);
TreeNode<Integer> node5 = new TreeNode<>(5);
TreeNode<Integer> node6 = new TreeNode<>(6);
node1.leftChild = node2;
node1.rightChild = node3;
node2.leftChild = node4;
node2.rightChild = node5;
node3.leftChild = node6;

DepthFirstSearch(node1, 6);