Stacks Written by J.J. Shepherd
Stacks This data structure works on the principle last in first out (LIFO) Any kind of pile is a good way to think of a stack A stack of games Pile of exams
Stacks Stacks can be represented by any simple abstract data structure Arrays Linked Lists There are two main pointers used while working with stacks Head: Points to the first element in the stack Tail: Points to the last element in the stack
Stacks There are four methods that are always in any representation of a stack Push: Adds a new element to the stack before the head Pop: Removes the element at the head of the stack (IE first element) Peek: Returns the value of the first element of the stack but does not remove it Print: Prints all elements in the queue
Arrays as Stacks The head is always index 0 The tail is the index of the first element inserted into the stack Index 1 2 3 4 5 6 Value 7 9 - Uninitialized Head Tail
Arrays as Stacks Push adds a new element to stack at the head Index 1 1 2 3 4 5 6 Value 7 9 - Uninitialized Head Tail
Arrays as Stacks First check whether or not the stack is full Index 1 if(tail >= data.length){return;} T tData = (T)aData; for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = tData; tail++; Index 1 2 3 4 5 6 Value 7 9 - Head Tail
Arrays as Stacks Starting from the tail shift all values right Index 1 if(tail >= data.length){return;} for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = (T)aData; tail++; Index 1 2 3 4 5 6 Value 7 9 - Head i Tail
Arrays as Stacks Starting from the tail shift all values right Index 1 if(tail >= data.length){return;} for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = (T)aData; tail++; Index 1 2 3 4 5 6 Value 7 9 - Head i Tail
Arrays as Stacks Starting from the tail shift all values right Index 1 if(tail >= data.length){return;} for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = (T)aData; tail++; Index 1 2 3 4 5 6 Value 7 9 - Head i Tail
Arrays as Stacks Starting from the tail shift all values right Index 1 if(tail >= data.length){return;} for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = (T)aData; tail++; Index 1 2 3 4 5 6 Value 7 9 - Head i Tail
Arrays as Stacks Starting from the tail shift all values right Index 1 if(tail >= data.length){return;} for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = (T)aData; tail++; Index 1 2 3 4 5 6 Value 7 9 - Head i Tail
Arrays as Stacks Starting from the tail shift all values right Index 1 if(tail >= data.length){return;} for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = (T)aData; tail++; Index 1 2 3 4 5 6 Value 7 9 - Head i Tail
ETC
Arrays as Stacks Starting from the tail shift all values right Index 1 if(tail >= data.length){return;} for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = (T)aData; tail++; Index 1 2 3 4 5 6 Value 7 9 - Head i Tail
Arrays as Stacks Set the head to the new data Index 1 2 3 4 5 6 Value if(tail >= data.length){return;} for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = (T)aData; tail++; Index 1 2 3 4 5 6 Value 69 7 9 - Head Tail
Arrays as Stacks Move the tail forward Index 1 2 3 4 5 6 Value 69 7 9 if(tail >= data.length){return;} for(int i = tail;i>0;i--) { data[i] = data[i-1]; } data[0] = (T)aData; tail++; Index 1 2 3 4 5 6 Value 69 7 9 - Head Tail
Example
Arrays as Stacks Pop removes the element at the head and returns it Index 1 2 3 4 5 6 Value 69 7 9 - Head Tail
Arrays as Stacks Return null if the array stack is uninitialized Index if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 69 7 9 - Head Tail
Arrays as Stacks Store the value at the head Index 1 2 3 4 5 6 Value if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 69 7 9 - retVal 69 Head Tail
Arrays as Stacks Starting from the head shift everything left Index 1 if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 69 7 9 - retVal 69 Head i Tail
Arrays as Stacks Starting from the head shift everything left Index 1 if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 7 9 - retVal 69 Head i Tail
Arrays as Stacks Starting from the head shift everything left Index 1 if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 7 9 - retVal 69 Head i Tail
Arrays as Stacks Starting from the head shift everything left Index 1 if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 7 9 - retVal 69 Head i Tail
Arrays as Stacks Starting from the head shift everything left Index 1 if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 7 9 - retVal 69 Head i Tail
Arrays as Stacks Starting from the head shift everything left Index 1 if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 7 9 - retVal 69 Head i Tail
ETC
Arrays as Stacks Starting from the head shift everything left Index 1 if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 7 9 - retVal 69 Head i Tail
Arrays as Stacks Move the tail back Index 1 2 3 4 5 6 Value 7 9 - Head if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 7 9 - retVal 69 Head Tail
Arrays as Stacks Return the value Index 1 2 3 4 5 6 Value 7 9 - Head if(data == null){return null;} T retVal = data[0]; for(int i=0;i<tail-1;i++) { data[i] = data[i+1]; } tail--; return retVal; Index 1 2 3 4 5 6 Value 7 9 - retVal 69 Head Tail
Example
Stacks with Linked Lists Hmm…
It’s Dumbly Easy