Add solution #640

Author: JoshCrozier

README.md

@@ -483,6 +483,7 @@
 637|[Average of Levels in Binary Tree](./0637-average-of-levels-in-binary-tree.js)|Easy|
 638|[Shopping Offers](./0638-shopping-offers.js)|Medium|
 639|[Decode Ways II](./0639-decode-ways-ii.js)|Hard|
+640|[Solve the Equation](./0640-solve-the-equation.js)|Medium|
 641|[Design Circular Deque](./0641-design-circular-deque.js)|Medium|
 643|[Maximum Average Subarray I](./0643-maximum-average-subarray-i.js)|Easy|
 645|[Set Mismatch](./0645-set-mismatch.js)|Medium|

solutions/0640-solve-the-equation.js

@@ -0,0 +1,55 @@
+/**
+ * 640. Solve the Equation
+ * https://leetcode.com/problems/solve-the-equation/
+ * Difficulty: Medium
+ *
+ * Solve a given equation and return the value of 'x' in the form of a string "x=#value".
+ * The equation contains only '+', '-' operation, the variable 'x' and its coefficient.
+ * You should return "No solution" if there is no solution for the equation, or "Infinite
+ * solutions" if there are infinite solutions for the equation.
+ *
+ * If there is exactly one solution for the equation, we ensure that the value of 'x' is
+ * an integer.
+ */
+
+/**
+ * @param {string} equation
+ * @return {string}
+ */
+var solveEquation = function(equation) {
+  const [left, right] = equation.split('=');
+  const leftSide = parseSide(left);
+  const rightSide = parseSide(right);
+  const totalX = leftSide.xCount - rightSide.xCount;
+  const totalNum = rightSide.numSum - leftSide.numSum;
+
+  if (totalX === 0 && totalNum === 0) return 'Infinite solutions';
+  if (totalX === 0) return 'No solution';
+
+  return `x=${totalNum / totalX}`;
+
+  function parseSide(side) {
+    let xCount = 0;
+    let numSum = 0;
+    let currentNum = 0;
+    let sign = 1;
+
+    for (let i = 0; i < side.length; i++) {
+      const char = side[i];
+      if (char === 'x') {
+        const v = i === 0 || side[i-1] === '+' || side[i-1] === '-';
+        xCount += sign * (currentNum === 0 && (v) ? 1 : currentNum);
+        currentNum = 0;
+      } else if (char === '+' || char === '-') {
+        numSum += sign * currentNum;
+        sign = char === '+' ? 1 : -1;
+        currentNum = 0;
+      } else {
+        currentNum = currentNum * 10 + parseInt(char);
+      }
+    }
+    numSum += sign * currentNum;
+
+    return { xCount, numSum };
+  }
+};