Merge pull request #884 from GangBean/main

[GangBean] Week 6

Author: GangBean

container-with-most-water/GangBean.java

@@ -0,0 +1,45 @@
+class Solution {
+    public int maxArea(int[] height) {
+        /**
+        1. understanding
+        - with two pair of line, each can contain "min(line1,line2) * (distance)" mount of water
+        - find maximum amount of water can contain
+        2. strategy
+        - brute force
+            - for each pair of lines, calculate amount and update maximum amount.
+            - it can takes O(N), where N is the count of lines.
+            - N is 10^5 at most, so it can takes 10^10, which can takes about 10 seconds
+        - you need to swap out unnecessary calculation
+        - so, let's find a unnecessary calculation in this problem.
+        3. complexity
+        - time: O(N)
+        - space: O(1)
+        */
+        int l = 0;
+        int r = height.length - 1;
+        int maxAmount = amountOf(height, l, r); // Math.min(height[l], height[r], r-l);
+        while (l < r) { // O(N)
+            maxAmount = Math.max(maxAmount, amountOf(height, l, r));
+            if (height[l] < height[r]) {
+                l++;
+            } else if (height[l] > height[r]) {
+                r--;
+            } else {
+                int nextLeftAmount = amountOf(height, l+1, r);
+                int nextRightAmount = amountOf(height, l, r-1);
+                if (nextLeftAmount < nextRightAmount) {
+                    r--;
+                } else {
+                    l++;
+                }
+            }
+        }
+
+        return maxAmount;
+    }
+
+    private int amountOf(int[] height, int left, int right) {
+        return (Math.min(height[left], height[right]) * (right - left));
+    }
+}
+

valid-parentheses/GangBean.java

@@ -0,0 +1,38 @@
+class Solution {
+    public boolean isValid(String s) {
+        /**
+        1. understanding
+        - given string contains character which has paired characters.
+        - validate all given characters in input string has each pair, and valid in order.
+        2. strategy
+        - use stack, to save left brackets, and if you encounter right bracket, then pop from stack, and compare two characters are paired.
+        - if not paired on any right character, or stack is empty then return false.
+        - all characters are iterated and stack is not empty, then return false.
+        3. complexity
+        - time: O(N), N is the length of input string s.
+        - space: O(N), for stack variable memory.
+        */
+        Stack<Character> leftBracket = new Stack<>();
+        for (char c : s.toCharArray()) {
+            if (c == '(' || c == '{' || c == '[') {
+                // when character is left bracket, then push to stack.
+                leftBracket.push(c);
+            } else if (c == ')' || c == '}' || c == ']') {
+                if (leftBracket.isEmpty()) return false;
+                char left = leftBracket.pop();
+                if (isPair(left, c)) continue;
+                return false;
+            } else {
+                throw new RuntimeException(String.format("Not valid input character: %c in input %s", c, s));
+            }
+        }
+        return leftBracket.isEmpty();
+    }
+
+    private boolean isPair(char left, char right) {
+        return (left == '(' && right == ')') 
+            || (left == '{' && right == '}')
+            || (left == '[' && right == ']');
+    }
+}
+