Update the section of intro to DP.

codes/cpp/chapter_dynamic_programming/climbing_stairs_constraint_dp.cpp

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+/**
+ * File: climbing_stairs_constraint_dp.cpp
+ * Created Time: 2023-07-01
+ * Author: Krahets (krahets@163.com)
+ */
+
+#include "../utils/common.hpp"
+
+/* 带约束爬楼梯：动态规划 */
+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
+    vector<vector<int>> dp(n + 1, vector<int>(3, 0));
+    // 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];
+}
+
+/* Driver Code */
+int main() {
+    int n = 9;
+
+    int res = climbingStairsConstraintDP(n);
+    cout << "爬 " << n << " 阶楼梯共有 " << res << " 种方案" << endl;
+
+    return 0;
+}