java——时间复杂度、动态数组
O(n)不一定小于O(n^2),要具体来看,而我们说的这种时间复杂度其实是渐进时间复杂度,描述的是n趋近于无穷的情况。
动态数组的时间复杂度:
添加操作:O(n) addLast()的均摊复杂度为O(1)
删除操作:O(n)
修改操作:已知索引:O(1) 未知索引:O(n)
查找操作:已知索引:O(1) 未知索引:O(n)
复杂度震荡:removeLast时resize过于着急(Eager)
解决方案:Lazy
实现动态数组:
public class Array<E> { //叫它静态数组 //private int[] data; private E[] data; private int size; //构造函数 public Array(int capacity) { data = (E[])new Object[capacity]; size = 0; } //无参数的构造函数,默认数组的容量为10 public Array() { this(10); }
public Array(E[] arr) {
//java不支持泛型数组:data = new E[];
data = (E[]) new Object[arr.length];
for(int i = 0 ; i < arr.length ; i ++)
data[i] = arr[i];
size = arr.length;
}
public int getSize() { return size; } public int getCapacity() { return data.length; } // O(1) public void addLast(E e) { add(size, e); } // O(n) public void addFirst(E e) { add(0, e); } // O(n/2) = O(n) public void add(int index, E e) { if(size>=data.length) resize(2 *data.length); if(index<0 || index>size) throw new IllegalArgumentException("Add failed.index is error."); for(int i=size-1;i>=index;i--) { data[i+1] = data[i]; } data[index] = e; size++; } @Override public String toString() { StringBuilder res = new StringBuilder(); res.append(String.format("Array: size = %d, capacity = %d\n", size, data.length)); res.append("["); for(int i = 0 ; i<size ; i++) { res.append(data[i]); if(i != size - 1) res.append(", "); } res.append("]"); return res.toString(); } E get(int index) { if(index < 0 || index >= size) throw new IllegalArgumentException("Get failed. Index is illegal"); return data[index]; } void set(int index, E e) { if(index < 0 || index >= size) throw new IllegalArgumentException("Get failed. Index is illegal"); data[index] = e; } public boolean contains(E e) { for(int i = 0; i < size; i++) { if(data[i].equals(e)) return true; } return false; } public int find(E e) { for(int i = 0; i < size; i++) { if(data[i].equals(e)) return i; } return -1; } public E remove(int index) { if(index < 0 || index >= size) throw new IllegalArgumentException("Get failed. Index is illegal"); E res = data[index]; for(int i = index; i<size; i++) { data[i] = data[i+1]; } size--; //释放空间,也可以不写 //loitering objects != memory leak data[size] = null; if(size == data.length / 4 && data.length / 2 != 0) resize(data.length / 2); return res; } public E removeFirst() { return remove(0); } public E removeLast() { return remove(size-1); } //只删除了一个e,并不能保证删除了全部e public void removeElement(E e) { int index = find(e); if(index != -1) remove(index); } private void resize(int newCapacity) { E[] newData = (E[]) new Object[newCapacity]; for(int i=0; i < size; i++) { newData[i] = data[i]; } data = newData; } }