finish 198, 200

Author: BaffinLee

101-200/198. House Robber.md

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+# 198. House Robber
+
+- Difficulty: Easy.
+- Related Topics: Dynamic Programming.
+- Similar Questions: Maximum Product Subarray, House Robber II, Paint House, Paint Fence, House Robber III, Non-negative Integers without Consecutive Ones, Coin Path, Delete and Earn.
+
+## Problem
+
+You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed, the only constraint stopping you from robbing each of them is that adjacent houses have security system connected and **it will automatically contact the police if two adjacent houses were broken into on the same night**.
+
+Given a list of non-negative integers representing the amount of money of each house, determine the maximum amount of money you can rob tonight **without alerting the police**.
+
+**Example 1:**
+
+```
+Input: [1,2,3,1]
+Output: 4
+Explanation: Rob house 1 (money = 1) and then rob house 3 (money = 3).
+             Total amount you can rob = 1 + 3 = 4.
+```
+
+**Example 2:**
+
+```
+Input: [2,7,9,3,1]
+Output: 12
+Explanation: Rob house 1 (money = 2), rob house 3 (money = 9) and rob house 5 (money = 1).
+             Total amount you can rob = 2 + 9 + 1 = 12.
+```
+
+## Solution 1
+
+```javascript
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var rob = function (nums) {
+  return helper([], 0, nums);
+};
+
+var helper = function (dp, i, nums) {
+  if (i >= nums.length) return 0;
+  if (dp[i] === undefined) {
+    dp[i] = Math.max(
+      helper(dp, i + 1, nums),
+      nums[i] + helper(dp, i + 2, nums)
+    );
+  }
+  return dp[i];
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).
+
+## Solution 2
+
+```javascript
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var rob = function (nums) {
+  var len = nums.length;
+  var dp = Array(len);
+  for (var i = len - 1; i >= 0; i--) {
+    dp[i] = Math.max(
+      dp[i + 1] || 0,
+      nums[i] + (dp[i + 2] || 0)
+    );
+  }
+  return dp[0] || 0;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).
+
+## Solution 3
+
+```javascript
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var rob = function (nums) {
+  var len = nums.length;
+  var dp = [0, 0];
+  for (var i = len - 1; i >= 0; i--) {
+    dp = [Math.max(dp[0], nums[i] + dp[1]), dp[0]];
+  }
+  return dp[0];
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(1).

101-200/200. Number of Islands.md

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+# 200. Number of Islands
+
+- Difficulty: Medium.
+- Related Topics: Depth-first Search, Breadth-first Search, Union Find.
+- Similar Questions: Surrounded Regions, Walls and Gates, Number of Islands II, Number of Connected Components in an Undirected Graph, Number of Distinct Islands, Max Area of Island.
+
+## Problem
+
+Given a 2d grid map of ```'1'```s (land) and ```'0'```s (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.
+
+**Example 1:**
+
+```
+Input:
+11110
+11010
+11000
+00000
+
+Output: 1
+```
+
+**Example 2:**
+
+```
+Input:
+11000
+11000
+00100
+00011
+
+Output: 3
+```
+
+## Solution
+
+```javascript
+/**
+ * @param {character[][]} grid
+ * @return {number}
+ */
+var numIslands = function(grid) {
+  var m = grid.length;
+  var n = (grid[0] || []).length;
+  var dp = Array(m).fill(0).map(_ => Array(n));
+  var num = 0;
+  for (var i = 0; i < n; i++) {
+    for (var j = 0; j < m; j++) {
+      if (dp[j][i] !== true && grid[j][i] === '1') {
+        num++;
+        mark(dp, j, i, grid);
+      }
+    }
+  }
+  return num;
+};
+
+var mark = function (dp, j, i, grid) {
+  if (dp[j] && dp[j][i] !== true && grid[j][i] === '1') {
+    dp[j][i] = true;
+    mark(dp, j - 1, i, grid);
+    mark(dp, j + 1, i, grid);
+    mark(dp, j, i - 1, grid);
+    mark(dp, j, i + 1, grid);
+  }
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(m*n).
+* Space complexity : O(m*n).