[YoungSeok-Choi] Week 9 Solutions

clone-graph/YoungSeok-Choi.java

@@ -0,0 +1,86 @@
+import java.util.HashMap;
+import java.util.Map;
+
+/*
+// Definition for a Node.
+class Node {
+    public int val;
+    public List<Node> neighbors;
+    public Node() {
+        val = 0;
+        neighbors = new ArrayList<Node>();
+    }
+    public Node(int _val) {
+        val = _val;
+        neighbors = new ArrayList<Node>();
+    }
+    public Node(int _val, ArrayList<Node> _neighbors) {
+        val = _val;
+        neighbors = _neighbors;
+    }
+}
+*/
+class Solution {
+    public Map<Node, Node> nMap = new HashMap<>();
+
+    public Node cloneGraph(Node node) {
+        if (node == null)
+            return null;
+
+        if (nMap.containsKey(node)) {
+            return nMap.get(node);
+        }
+
+        Node clone = new Node(node.val);
+        nMap.put(node, clone);
+
+        for (Node nei : node.neighbors) {
+            clone.neighbors.add(cloneGraph(nei));
+        }
+
+        return clone;
+    }
+}
+
+class WrongSolution {
+    public Node graph;
+    public Map<Node, Node> nMap = new HashMap<>();
+
+    public Node cloneGraph(Node node) {
+        if (node == null)
+            return null;
+
+        graph = new Node(node.val);
+        nMap.put(node, graph);
+
+        // print(graph);
+
+        return clone(node, graph);
+    }
+
+    public Node clone(Node node, Node cur) {
+        for (int i = 0; i < node.neighbors.size(); i++) {
+            Node adj = node.neighbors.get(i);
+
+            if (nMap.containsKey(adj)) {
+                return nMap.get(adj);
+            }
+
+            nMap.put(adj, new Node(adj.val));
+            cur.neighbors.add(nMap.get(adj));
+            clone(adj, nMap.get(adj));
+        }
+
+        return cur;
+    }
+
+    public void print(Node node) {
+        System.out.println("visit " + node.val);
+
+        for (int i = 0; i < node.neighbors.size(); i++) {
+            Node adj = node.neighbors.get(i);
+            System.out.println("nei " + adj.val);
+            print(adj);
+        }
+    }
+}

linked-list-cycle/YoungSeok-Choi.java

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+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+public class Solution {
+    public boolean hasCycle(ListNode head) {
+        Set<ListNode> nSet = new HashSet<>();
+
+        while (head != null) {
+            if (nSet.contains(head)) {
+                return true;
+            }
+
+            nSet.add(head);
+            head = head.next;
+        }
+
+        return false;
+    }
+}
+
+/**
+ * Definition for singly-linked list.
+ * class ListNode {
+ * int val;
+ * ListNode next;
+ * ListNode(int x) {
+ * val = x;
+ * next = null;
+ * }
+ * }
+ */
+// NOTE: 같은 val의 다른 Node의 경우를 고려하지 못했다
+class WrongSolution {
+    public boolean hasCycle(ListNode head) {
+        Map<Integer, List<Integer>> vMap = new HashMap<>(); // NOTE: val, pos
+
+        if (head == null) {
+            return false;
+        }
+
+        ListNode cur = head;
+        boolean isCyclick = false;
+        int pos = -1;
+        while (cur != null) {
+
+            if (vMap.containsKey(cur.val)) {
+                isCyclick = true;
+                break;
+            }
+
+            List<Integer> posList = new ArrayList<>();
+            posList.add(pos++);
+            vMap.put(cur.val, posList);
+            cur = head.next;
+        }
+
+        if (isCyclick) {
+            return true;
+        } else {
+            return false;
+        }
+    }
+}

longest-common-subsequence/YoungSeok-Choi.java

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+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+class Solution {
+    private Map<String, Integer> memo = new HashMap<>();
+
+    public int longestCommonSubsequence(String t1, String t2) {
+        return lcs(0, 0, t1, t2);
+    }
+
+    private int lcs(int i, int j, String t1, String t2) {
+        if (i == t1.length() || j == t2.length()) {
+            return 0;
+        }
+
+        String key = i + "," + j;
+        if (memo.containsKey(key)) {
+            return memo.get(key);
+        }
+
+        int res;
+        if (t1.charAt(i) == t2.charAt(j)) {
+            res = 1 + lcs(i + 1, j + 1, t1, t2);
+        } else {
+            res = Math.max(lcs(i + 1, j, t1, t2), lcs(i, j + 1, t1, t2));
+        }
+
+        memo.put(key, res);
+        return res;
+    }
+}
+
+class WrongSolution {
+    public int longestCommonSubsequence(String t1, String t2) {
+        Map<Integer, List<Integer>> sMap = new HashMap<>();
+        int cnt = 0;
+
+        // t1의 연속으로 등장하는 문자열을 하나로만 바꾸기 (e.g. abbbvvvvssssmmmddd -> abvsmd)
+        StringBuilder sb = new StringBuilder();
+        char start = t1.charAt(0);
+        sb.append(start);
+        for (int i = 1; i < t1.length(); i++) {
+            if (start != t1.charAt(i)) {
+                sb.append(t1.charAt(i));
+                start = t1.charAt(i);
+            }
+        }
+
+        t1 = sb.toString();
+
+        for (int i = 0; i < t1.length(); i++) {
+            sMap.computeIfAbsent(t1.charAt(i) - 97, k -> new ArrayList<>()).add(i);
+        }
+
+        StringBuilder filtered = new StringBuilder();
+        for (int i = 0; i < t2.length(); i++) {
+            int alpIdx = t2.charAt(i) - 97;
+            if (sMap.containsKey(alpIdx)) {
+                filtered.append(t2.charAt(i));
+            }
+        }
+        t2 = filtered.toString();
+
+        int prevT1Idx = -1;
+        for (int i = 0; i < t2.length(); i++) {
+            int alpIdx = t2.charAt(i) - 97;
+
+            List<Integer> idxList = sMap.get(alpIdx);
+            if (idxList == null)
+                continue;
+
+            for (int idx : idxList) {
+                if (idx > prevT1Idx) {
+                    cnt++;
+                    prevT1Idx = idx;
+                    break;
+                }
+            }
+        }
+
+        return cnt;
+    }
+}

maximum-product-subarray/YoungSeok-Choi.java

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+import java.util.ArrayList;
+import java.util.List;
+
+// 답지보고 풀이..
+class Solution {
+    public int maxProduct(int[] nums) {
+        int max = nums[0];
+        int currMax = nums[0];
+        int currMin = nums[0];
+
+        for (int i = 1; i < nums.length; i++) {
+            int temp = currMax;
+            currMax = Math.max(nums[i], Math.max(currMax * nums[i], currMin * nums[i]));
+            currMin = Math.min(nums[i], Math.min(temp * nums[i], currMin * nums[i]));
+            max = Math.max(max, currMax);
+        }
+
+        return max;
+    }
+}
+
+// NOTE:핵심 컨셉은 0을 만나기 전까지 한칸씩 늘려가며 모든 원소의 곱을 만든다. 0을 만나면 첫 번째 원소를 뺀 나머지 원소들의 곱의
+// 값으로 최대값을 계속 갱신해나간다.
+
+// 위의 아이디어로 문제를 풀이했지만, 모든 경우의 예외를 핸들링 할 수가 없어 풀이를 더이상 진행하지 않았다..
+class WrongSolution {
+
+    public int mx = -98764321;
+    public List<Boolean> bList = new ArrayList<>();
+    public boolean isStartInit = true;
+    public boolean isOverZero = false;
+
+    public int maxProduct(int[] nums) {
+        int start = 1;
+        int zeroIdx = 0;
+        if (nums.length == 1) {
+            return nums[0];
+        }
+
+        for (int i = 0; i < nums.length; i++) {
+
+            if (nums[i] == 0) {
+                if (mx <= 0) {
+                    bList.add(false);
+                } else {
+                    bList.add(true);
+                }
+
+                while (zeroIdx < i) {
+                    start /= nums[zeroIdx];
+                    mx = Math.max(mx, start);
+
+                    if (mx >= 1) {
+                        bList.add(true);
+                    }
+
+                    zeroIdx++;
+                }
+
+                start = 1;
+                isStartInit = true;
+                zeroIdx++; // 0값인 인덱스를 건너뛰고 다음 번 인덱스를 대입
+            } else {
+                start *= nums[i];
+                mx = Math.max(mx, start);
+
+                if (start >= 1) {
+                    isStartInit = false;
+                    isOverZero = true;
+                }
+            }
+        }
+
+        while (zeroIdx < nums.length) {
+            start /= nums[zeroIdx];
+            mx = Math.max(mx, start);
+            zeroIdx++;
+        }
+
+        mx = Math.max(mx, nums[nums.length - 1]);
+
+        if (nums[nums.length - 1] >= 1) {
+            isOverZero = true;
+        }
+
+        if (mx > 0 && isOverZero) {
+            bList.add(true);
+        }
+
+        for (boolean b : bList) {
+            if (b) {
+                return mx;
+            }
+        }
+
+        if (bList.size() > 0) {
+            return isStartInit ? 0 : mx;
+        }
+
+        return mx;
+    }
+}