finish 134

Author: BaffinLee

134. Gas Station.js

@@ -0,0 +1,36 @@
+/**
+ * 134. Gas Station
+ * 
+ * There are N gas stations along a circular route, where the amount of gas at station i is gas[i].
+ * 
+ * You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from station i to its next station (i+1). You begin the journey with an empty tank at one of the gas stations.
+ * 
+ * Return the starting gas station's index if you can travel around the circuit once, otherwise return -1.
+ * 
+ * Note:
+ * The solution is guaranteed to be unique.
+ */
+
+/**
+ * @param {number[]} gas
+ * @param {number[]} cost
+ * @return {number}
+ */
+var canCompleteCircuit = function(gas, cost) {
+	var len = gas.length;
+	var tank = 0;
+	var total = 0;
+	var start = 0;
+	for (var i = 0; i < len; i++) {
+		tank += gas[i] - cost[i];
+		if (tank < 0) {
+			start = i + 1;
+			total += tank;
+			tank = 0;
+		}
+	}
+	return tank + total >= 0 ? start : -1;
+};
+
+// n 点到不了 m 点，则其间所有点都到不了 m 点
+// 总和大于等于 0 说明有解