[선재] WEEK 03

climbing-stairs/sunjae95.js

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+/**
+ * @description
+ * brainstorming:
+ * Dynamic Programming
+ *
+ * time complexity: O(n)
+ * space complexity: O(n)
+ */
+
+var climbStairs = function (n) {
+  const dp = Array.from({ length: n + 1 }, () => 0);
+  dp[1] = 1;
+  dp[2] = 2;
+
+  for (let i = 3; i < n + 1; i++) dp[i] = dp[i - 1] + dp[i - 2];
+
+  return dp[n];
+};

coin-change/sunjae95.js

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+/**
+ * @description
+ * brainstorming:
+ * 1. asc sort + division calculate
+ * 2. bfs + memoization
+ *
+ * strategy:
+ * bfs + memoization
+ *
+ * reason:
+ * Tried with brainstorming 1 but test case is false
+ *
+ * time complexity: O(n^k)
+ * space complexity: O(n)
+ */
+class Node {
+  constructor(val) {
+    this.value = val;
+    this.next = null;
+  }
+}
+
+class CustomQueue {
+  constructor() {
+    this.front = null;
+    this.rear = null;
+    this.size = 0;
+  }
+
+  push(val) {
+    const node = new Node(val);
+
+    if (this.size === 0) {
+      this.front = node;
+      this.rear = node;
+    } else {
+      this.rear.next = node;
+      this.rear = node;
+    }
+
+    this.size++;
+  }
+
+  pop() {
+    if (this.size === 0) return null;
+    const node = this.front;
+    this.front = this.front.next;
+    this.size--;
+    if (this.size === 0) this.rear = null;
+
+    return node.value;
+  }
+}
+
+var coinChange = function (coins, amount) {
+  const queue = new CustomQueue();
+  const memoSet = new Set();
+
+  if (amount === 0) return 0;
+
+  for (const coin of coins) {
+    if (amount === coin) return 1;
+
+    queue.push(coin);
+    memoSet.add(coin);
+  }
+
+  let count = 1;
+
+  while (queue.size) {
+    count++;
+    let depthSize = queue.size;
+
+    while (depthSize--) {
+      const sum = queue.pop();
+
+      for (const coin of coins) {
+        const nextSum = sum + coin;
+
+        if (memoSet.has(nextSum)) continue;
+        if (amount === nextSum) return count;
+        if (amount > nextSum) queue.push(nextSum);
+
+        memoSet.add(nextSum);
+      }
+    }
+  }
+
+  return -1;
+};

combination-sum/sunjae95.js

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+/**
+ * @description
+ * brainstorming:
+ * dfs
+ *
+ * time complexity: O(n^k)
+ * space complexity: O(n)
+ */
+var combinationSum = function (candidates, target) {
+  const answer = [];
+
+  const dfs = (array, sum, index) => {
+    if (sum > target) return;
+    if (sum === target) return answer.push(array);
+
+    for (let i = index; i < candidates.length; i++) {
+      const nextArray = array.concat(candidates[i]);
+      const nextSum = sum + candidates[i];
+
+      dfs(nextArray, nextSum, i);
+    }
+  };
+
+  candidates.forEach((value, i) => dfs([value], value, i));
+
+  return answer;
+};

product-of-array-except-self/sunjae95.js

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+/**
+ * @description
+ * brainstorming:
+ * recursive function
+ *
+ * time complexity: O(n)
+ * space complexity: O(n)
+ */
+var productExceptSelf = function (nums) {
+  const answer = Array.from({ length: nums.length }, () => 0);
+
+  const search = (left, right, i) => {
+    if (i === nums.length - 1) {
+      answer[i] = left * right;
+      return nums[i];
+    }
+
+    const productLeft = left * nums[i];
+    const productRight = search(productLeft, right, i + 1);
+
+    answer[i] = left * productRight;
+
+    return productRight * nums[i];
+  };
+
+  search(1, 1, 0);
+
+  return answer;
+};

two-sum/sunjae95.js

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+/**
+ * @description
+ * brainstorming:
+ * 1. O(n^2) brute force
+ * 2. hash table
+ */
+
+/**
+ * @description brainstorming 1 solve
+ * time complexity: O(n^2)
+ * space complexity: O(1)
+ */
+var twoSum = function (nums, target) {
+  for (let i = 0; i < nums.length; i++) {
+    for (let j = i + 1; j < nums.length; j++) {
+      if (nums[i] + nums[j] === target) return [i, j];
+    }
+  }
+};
+
+/**
+ * @description brainstorming 2 solve
+ * time complexity: O(n^2)
+ * space complexity: O(n)
+ */
+var twoSum = function (nums, target) {
+  const map = new Map();
+
+  nums.forEach((num, index) => {
+    if (!map.get(num)) return map.set(num, [index]);
+
+    map.set(num, map.get(num).concat(index));
+  });
+
+  for (let i = 0; i < nums.length; i++) {
+    const rest = target - nums[i];
+
+    if (!map.get(rest)) continue;
+
+    const indexList = map.get(rest);
+    for (let j = 0; j < indexList.length; j++) {
+      if (i === indexList[j]) continue;
+
+      return [i, indexList[j]];
+    }
+  }
+};