update P2602.md

Author: coderhare

Diff for: luogu/P2602.md

@@ -0,0 +1,32 @@
+计数DP模板题
+```c++
+void solve(){
+    //[a,b]中所有数码出现次数
+    ll x, y, cnt;
+    cin >> x >> y;
+    ll A[20], dp[20][20][2][2], digit;
+    auto dfs = [&](auto & dfs, ll cntd, int pos, bool limit, bool lead){
+        if(pos < 0) return cntd;
+        if(dp[cntd][pos][limit][lead] != -1) return dp[cntd][pos][limit][lead];
+        ll ans = 0;
+        for(int v = 0; v <= (limit? A[pos]: 9); v++){
+            if(lead && v == 0)
+                ans += dfs(dfs, cntd, pos - 1, limit && v == A[pos], true);
+            else
+                ans += dfs(dfs, cntd + (v == digit), pos - 1, limit && v == A[pos], false);
+        }
+        return dp[cntd][pos][limit][lead] = ans;
+    };
+    auto f = [&](ll x){
+        cnt = 0;
+        memset(A, 0, sizeof A);
+        memset(dp, -1, sizeof dp);
+        while(x) A[cnt++] = x % 10, x /= 10;
+        return dfs(dfs, 0, cnt - 1, true, true);
+    };
+    for(int i = 0; i <= 9; i++){
+        digit = i;
+        cout << f(y) - f(x - 1) << " \n"[i == 9];
+    }
+}
+```