Add the chapter of backtracking. (#459)

codes/java/chapter_backtracking/preorder_find_paths.java

@@ -0,0 +1,50 @@
+/**
+ * File: preorder_find_paths.java
+ * Created Time: 2023-04-16
+ * Author: Krahets (krahets@163.com)
+ */
+
+import include.*;
+import java.util.*;
+
+public class preorder_find_paths {
+    static List<TreeNode> path;
+    static List<List<TreeNode>> res;
+
+    /* 前序遍历 */
+    static void preOrder(TreeNode root) {
+        if (root == null) {
+            return;
+        }
+        // 尝试
+        path.add(root);
+        if (root.val == 7) {
+            // 记录解
+            res.add(new ArrayList<>(path));
+        }
+        preOrder(root.left);
+        preOrder(root.right);
+        // 回退
+        path.remove(path.size() - 1);
+    }
+
+    public static void main(String[] args) {
+        TreeNode root = TreeNode.listToTree(Arrays.asList(1, 7, 3, 4, 5, 6, 7));
+        System.out.println("\n初始化二叉树");
+        PrintUtil.printTree(root);
+
+        // 前序遍历
+        path = new ArrayList<>();
+        res = new ArrayList<>();
+        preOrder(root);
+
+        System.out.println("\n输出所有根节点到节点 7 的路径");
+        for (List<TreeNode> path : res) {
+            List<Integer> vals = new ArrayList<>();
+            for (TreeNode node : path) {
+                vals.add(node.val);
+            }
+            System.out.println(vals);
+        }
+    }
+}