Update the section of intro to DP.

codes/java/chapter_dynamic_programming/climbing_stairs_constraint_dp.java

@@ -0,0 +1,36 @@
+/**
+ * File: climbing_stairs_constraint_dp.java
+ * Created Time: 2023-07-01
+ * Author: Krahets (krahets@163.com)
+ */
+
+package chapter_dynamic_programming;
+
+public class climbing_stairs_constraint_dp {
+    /* 带约束爬楼梯：动态规划 */
+    static int climbingStairsConstraintDP(int n) {
+        if (n == 1 || n == 2) {
+            return n;
+        }
+        // 初始化 dp 列表，用于存储子问题的解
+        int[][] dp = new int[n + 1][3];
+        // 初始状态：预设最小子问题的解
+        dp[1][1] = 1;
+        dp[1][2] = 0;
+        dp[2][1] = 0;
+        dp[2][2] = 1;
+        // 状态转移：从较小子问题逐步求解较大子问题
+        for (int i = 3; i <= n; i++) {
+            dp[i][1] = dp[i - 1][2];
+            dp[i][2] = dp[i - 2][1] + dp[i - 2][2];
+        }
+        return dp[n][1] + dp[n][2];
+    }
+
+    public static void main(String[] args) {
+        int n = 9;
+
+        int res = climbingStairsConstraintDP(n);
+        System.out.println(String.format("爬 %d 阶楼梯共有 %d 种方案", n, res));
+    }
+}