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# Recursividade

```java
public static String invertir(String palabra) {
    // CASO BASE
    if (palabra.length() <= 1)
        return palabra;

    // CASO RECURSIVO
    return invertir(palabra.substring(1)) + palabra.charAt(0);
}
```

```java
public static int suma(int n) throws Exception{
    // Si n es negativo, podría lanzar excepción
    if (n <= 0) throw new Exception("Numero negativo no soportado"
    if (n == 1) return 1;
    return n + suma(n - 1);
}
```

```java
public static int contaCifras(int numero, int cifra) {
    
    
    // CASO BASE: cando xa non quedan díxitos
    if (numero == 0) {
        return 0;
    }
    
    // Comprobamos o último díxito
    int ultima = numero % 10;

    // CASO RECURSIVO: sumamos 1 se coincide e seguimos co resto
    if (ultima == cifra)
        return 1 + contaCifras(numero / 10, cifra);
    else
        return contaCifras(numero / 10, cifra);
}
```

```java
public static int potencia(int base, int exp) throws Exception {
    // CASO BASE 1: exponente 0 (cualquier número^0 = 1)
    if (exp == 0) {
        return 1;
    }
    
    // CASO BASE 2: exponente 1 (cualquier número^1 = sí mismo)
    if (exp == 1) {
        return base;
    }
    
    // CASO BASE 3: exponente negativo (no manejado, podríamos lanzar excepción)
    if (exp < 0) {
        // Podríamos hacer: return 1 / potencia(base, -exp) pero sería double
        throw new Exception("Exponente negativo no soportado");
    }
    
    // CASO RECURSIVO: base^exp = base * base^(exp-1)
    return base * potencia(base, exp - 1);
}
```

```java
public static boolean palindromo(String palabra){
        int len = palabra.length();
        if (len==0 || len ==1)
            return true;
        else if (palabra.charAt(0)!=palabra.charAt(len-1)){
            return false;
        } else {
            return palindromo(palabra.substring(1,len-1));
        }
    }
```


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