solve problem

leet-code/67-binary-add/README.md

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+# 67-binary-add
+Given two binary strings a and b, return their sum as a binary string.
+
+Example 1:
+
+Input: a = "11", b = "1"
+Output: "100"
+Example 2:
+
+Input: a = "1010", b = "1011"
+Output: "10101"
+
+
+Constraints:
+
+1 <= a.length, b.length <= 104
+a and b consist only of '0' or '1' characters.
+Each string does not contain leading zeros except for the zero itself.

leet-code/67-binary-add/index.js

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+/**
+ * @param {string} a
+ * @param {string} b
+ * @return {string}
+ */
+var addBinary = function(a, b) {
+    // pointer for each bitstring
+    let i = a.length - 1;
+    let j = b.length - 1;
+    // store the carry value for each place value
+    let carry = 0
+    // store the resulting bitstring
+    const result = []
+    // continue as long as either input bitstring has digits remaining
+    while (i >= 0 || j >= 0) {
+        // read the next digit in each bitstring
+        const a1 = a[i] === '1' ? 1 : 0
+        const b1 = b[j] === '1' ? 1 : 0
+        // sum of digits
+        const sum = carry + a1 + b1
+        // divide by 2
+        carry = sum >> 1
+        // remainder upon division by 2
+        result.push(sum & 1)
+        // progress to next digit
+        i--
+        j--
+    }
+    // add left over carry as most significant digit
+    if (carry) result.push('1');
+    // order most to least significant bits
+    return result.reverse().join(''); 
+};