895

Author: lzl124631x

README.md

@@ -361,7 +361,7 @@ Now I'm using a Chrome Extension I developed -- [LeetCoder](https://chrome.googl
 414 | Third Maximum Number |  | [Solution](leetcode/414.%20Third%20Maximum%20Number)
 415 | Add Strings | Easy | [Solution](leetcode/415.%20Add%20Strings)
 416 | Partition Equal Subset Sum | Medium | [Solution](leetcode/416.%20Partition%20Equal%20Subset%20Sum)
-417 | Pacific Atlantic Water Flow |  | [Solution](leetcode/417.%20Pacific%20Atlantic%20Water%20Flow)
+417 | Pacific Atlantic Water Flow | Medium | [Solution](leetcode/417.%20Pacific%20Atlantic%20Water%20Flow)
 419 | Battleships in a Board |  | [Solution](leetcode/419.%20Battleships%20in%20a%20Board)
 420 | Strong Password Checker |  | [Solution](leetcode/420.%20Strong%20Password%20Checker)
 421 | Maximum XOR of Two Numbers in an Array | Medium | [Solution](leetcode/421.%20Maximum%20XOR%20of%20Two%20Numbers%20in%20an%20Array)
@@ -715,6 +715,7 @@ Now I'm using a Chrome Extension I developed -- [LeetCoder](https://chrome.googl
 892 | Surface Area of 3D Shapes | Easy | [Solution](leetcode/892.%20Surface%20Area%20of%203D%20Shapes)
 893 | Groups of Special-Equivalent Strings | Easy | [Solution](leetcode/893.%20Groups%20of%20Special-Equivalent%20Strings)
 894 | All Possible Full Binary Trees | Medium | [Solution](leetcode/894.%20All%20Possible%20Full%20Binary%20Trees)
+895 | Maximum Frequency Stack | Hard | [Solution](leetcode/895.%20Maximum%20Frequency%20Stack)
 896 | Monotonic Array | Easy | [Solution](leetcode/896.%20Monotonic%20Array)
 897 | Increasing Order Search Tree | Easy | [Solution](leetcode/897.%20Increasing%20Order%20Search%20Tree)
 899 | Orderly Queue | Hard | [Solution](leetcode/899.%20Orderly%20Queue)

leetcode/895. Maximum Frequency Stack/README.md

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+# [895. Maximum Frequency Stack (Hard)](https://leetcode.com/problems/maximum-frequency-stack/)
+
+<p>Design a stack-like data structure to push elements to the stack and pop the most frequent element from the stack.</p>
+
+<p>Implement the <code>FreqStack</code> class:</p>
+
+<ul>
+	<li><code>FreqStack()</code> constructs an empty frequency stack.</li>
+	<li><code>void push(int val)</code> pushes an integer <code>val</code> onto the top of the stack.</li>
+	<li><code>int pop()</code> removes and returns the most frequent element in the stack.
+	<ul>
+		<li>If there is a tie for the most frequent element, the element closest to the stack's top is removed and returned.</li>
+	</ul>
+	</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre><strong>Input</strong>
+["FreqStack", "push", "push", "push", "push", "push", "push", "pop", "pop", "pop", "pop"]
+[[], [5], [7], [5], [7], [4], [5], [], [], [], []]
+<strong>Output</strong>
+[null, null, null, null, null, null, null, 5, 7, 5, 4]
+
+<strong>Explanation</strong>
+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].
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>0 &lt;= val &lt;= 10<sup>9</sup></code></li>
+	<li>At most <code>2 * 10<sup>4</sup></code> calls will be made to <code>push</code> and <code>pop</code>.</li>
+	<li>It is guaranteed that there will be at least one element in the stack before calling <code>pop</code>.</li>
+</ul>
+
+
+**Related Topics**:  
+[Hash Table](https://leetcode.com/tag/hash-table/), [Stack](https://leetcode.com/tag/stack/)
+
+## Solution 1.
+
+We push elements based on frequency first then the order they pushed into the stack.
+
+Consider input `5 7 5 7 4 5`, if we label their frequency at the top
+
+```
+1 1 2 2 1 3
+5 7 5 7 4 5
+```
+
+We can reorder them in this way
+
+```
+1: 5 7 4
+2: 5 7
+3: 5
+```
+
+When popping elements, we pop the 3rd stack first, then the 2nd, then the 1st.
+
+```cpp
+// OJ: https://leetcode.com/problems/maximum-frequency-stack/
+// Author: github.com/lzl124631x
+// Time: O(1) for all
+// Space: O(N)
+class FreqStack {
+    unordered_map<int, int> freq;
+    vector<vector<int>> v;
+public:
+    FreqStack() {}
+    void push(int val) {
+        int f = freq[val]++;
+        if (v.size() <= f) v.emplace_back();
+        v[f].push_back(val);
+    }
+    int pop() {
+        int val = v.back().back();
+        v.back().pop_back();
+        --freq[val];
+        if (v.back().empty()) v.pop_back();
+        return val;
+    }
+};
+```