Maximum Frequency Stack

Author: volodimyr

895.maximum_frequency_stack/stack.go

@@ -0,0 +1,87 @@
+// 895. Maximum Frequency Stack
+// Topics: 'Array', 'Stack'
+// Level: 'Hard'
+
+// Design a stack-like data structure to push elements to the stack and pop the most frequent element from the stack.
+
+// Implement the FreqStack class:
+
+//     FreqStack() constructs an empty frequency stack.
+//     void push(int val) pushes an integer val onto the top of the stack.
+//     int pop() removes and returns the most frequent element in the stack.
+//         If there is a tie for the most frequent element, the element closest to the stack's top is removed and returned.
+
+// Example 1:
+
+// Input
+// ["FreqStack", "push", "push", "push", "push", "push", "push", "pop", "pop", "pop", "pop"]
+// [[], [5], [7], [5], [7], [4], [5], [], [], [], []]
+// Output
+// [null, null, null, null, null, null, null, 5, 7, 5, 4]
+
+// Explanation
+// FreqStack freqStack = new FreqStack();
+// freqStack.push(5); // The stack is [5]
+// freqStack.push(7); // The stack is [5,7]
+// freqStack.push(5); // The stack is [5,7,5]
+// freqStack.push(7); // The stack is [5,7,5,7]
+// freqStack.push(4); // The stack is [5,7,5,7,4]
+// freqStack.push(5); // The stack is [5,7,5,7,4,5]
+// freqStack.pop();   // return 5, as 5 is the most frequent. The stack becomes [5,7,5,7,4].
+// freqStack.pop();   // return 7, as 5 and 7 is the most frequent, but 7 is closest to the top. The stack becomes [5,7,5,4].
+// freqStack.pop();   // return 5, as 5 is the most frequent. The stack becomes [5,7,4].
+// freqStack.pop();   // return 4, as 4, 5 and 7 is the most frequent, but 4 is closest to the top. The stack becomes [5,7].
+
+// Constraints:
+
+//     0 <= val <= 109
+//     At most 2 * 104 calls will be made to push and pop.
+//     It is guaranteed that there will be at least one element in the stack before calling pop.
+
+package maximumfrequencystack
+
+type FreqStack struct {
+	max    int
+	stacks map[int][]int
+	freq   map[int]int
+}
+
+func Constructor() FreqStack {
+	return FreqStack{
+		max:    1,
+		freq:   map[int]int{},
+		stacks: map[int][]int{},
+	}
+}
+
+func (f *FreqStack) Push(val int) {
+	fq, ok := f.freq[val]
+	if !ok {
+		fq = 1
+	} else {
+		fq++
+	}
+	f.freq[val] = fq
+	stack, ok := f.stacks[fq]
+	if !ok {
+		stack = []int{val}
+	} else {
+		stack = append(stack, val)
+	}
+	f.stacks[fq] = stack
+	if fq > f.max {
+		f.max = fq
+	}
+}
+
+func (f *FreqStack) Pop() int {
+	stack := f.stacks[f.max]
+	v := stack[len(stack)-1]
+	f.stacks[f.max] = stack[:len(stack)-1]
+	if len(f.stacks[f.max]) == 0 {
+		delete(f.stacks, f.max)
+		f.max--
+	}
+	f.freq[v]--
+	return v
+}

895.maximum_frequency_stack/stack_test.go

@@ -0,0 +1,130 @@
+package maximumfrequencystack
+
+import (
+	"testing"
+)
+
+func TestFreqStack(t *testing.T) {
+	freqStack := Constructor()
+	freqStack.Push(5)
+	freqStack.Push(7)
+	freqStack.Push(5)
+	freqStack.Push(7)
+	freqStack.Push(4)
+	freqStack.Push(5)
+
+	if got := freqStack.Pop(); got != 5 {
+		t.Errorf("Pop() = %v, want %v", got, 5)
+	}
+	if got := freqStack.Pop(); got != 7 {
+		t.Errorf("Pop() = %v, want %v", got, 7)
+	}
+	if got := freqStack.Pop(); got != 5 {
+		t.Errorf("Pop() = %v, want %v", got, 5)
+	}
+	if got := freqStack.Pop(); got != 4 {
+		t.Errorf("Pop() = %v, want %v", got, 4)
+	}
+}
+
+func TestFreqStackSingleElement(t *testing.T) {
+	freqStack := Constructor()
+	freqStack.Push(1)
+
+	if got := freqStack.Pop(); got != 1 {
+		t.Errorf("Pop() = %v, want %v", got, 1)
+	}
+}
+
+func TestFreqStackAllSameElement(t *testing.T) {
+	freqStack := Constructor()
+	freqStack.Push(3)
+	freqStack.Push(3)
+	freqStack.Push(3)
+
+	if got := freqStack.Pop(); got != 3 {
+		t.Errorf("Pop() = %v, want %v", got, 3)
+	}
+	if got := freqStack.Pop(); got != 3 {
+		t.Errorf("Pop() = %v, want %v", got, 3)
+	}
+	if got := freqStack.Pop(); got != 3 {
+		t.Errorf("Pop() = %v, want %v", got, 3)
+	}
+}
+
+func TestFreqStackMultipleOperations(t *testing.T) {
+	freqStack := Constructor()
+	freqStack.Push(1)
+	freqStack.Push(2)
+	freqStack.Push(1)
+
+	if got := freqStack.Pop(); got != 1 {
+		t.Errorf("Pop() = %v, want %v", got, 1)
+	}
+
+	freqStack.Push(3)
+	freqStack.Push(2)
+
+	if got := freqStack.Pop(); got != 2 {
+		t.Errorf("Pop() = %v, want %v", got, 2)
+	}
+	if got := freqStack.Pop(); got != 3 {
+		t.Errorf("Pop() = %v, want %v", got, 3)
+	}
+	if got := freqStack.Pop(); got != 2 {
+		t.Errorf("Pop() = %v, want %v", got, 2)
+	}
+	if got := freqStack.Pop(); got != 1 {
+		t.Errorf("Pop() = %v, want %v", got, 1)
+	}
+}
+
+func TestFreqStackTieBreaker(t *testing.T) {
+	freqStack := Constructor()
+	freqStack.Push(1)
+	freqStack.Push(2)
+	freqStack.Push(3)
+
+	if got := freqStack.Pop(); got != 3 {
+		t.Errorf("Pop() = %v, want %v", got, 3)
+	}
+	if got := freqStack.Pop(); got != 2 {
+		t.Errorf("Pop() = %v, want %v", got, 2)
+	}
+	if got := freqStack.Pop(); got != 1 {
+		t.Errorf("Pop() = %v, want %v", got, 1)
+	}
+}
+
+func TestFreqStackComplexScenario(t *testing.T) {
+	freqStack := Constructor()
+	freqStack.Push(4)
+	freqStack.Push(0)
+	freqStack.Push(9)
+	freqStack.Push(3)
+	freqStack.Push(4)
+	freqStack.Push(2)
+
+	if got := freqStack.Pop(); got != 4 {
+		t.Errorf("Pop() = %v, want %v", got, 4)
+	}
+
+	freqStack.Push(6)
+
+	if got := freqStack.Pop(); got != 6 {
+		t.Errorf("Pop() = %v, want %v", got, 6)
+	}
+
+	freqStack.Push(1)
+
+	if got := freqStack.Pop(); got != 1 {
+		t.Errorf("Pop() = %v, want %v", got, 1)
+	}
+
+	freqStack.Push(1)
+
+	if got := freqStack.Pop(); got != 1 {
+		t.Errorf("Pop() = %v, want %v", got, 1)
+	}
+}