公式： f(x)=max(0,x)
# What you gain
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The Cardiff-born star moved to the US to pursue a career in Hollywood, and now lives in New York with his wife, actress Keri Russell.，详情可参考91视频
// 栈空 → 无更大元素，返回-1；栈非空 → 取栈顶（第一个更大值）